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			{{Short description\|Branch of philosophy}}
	'''Perception''' is one of our most important mental processes. If we could not perceive anything, then we would know nothing except the contents of our own minds. Because it is our window onto the world, it is important for us to know some basic facts about perception. For exmple, does our perception let us experience the world as it really is? What are the immediate objects of perception? The '''philosophy of perception''' tackles these difficult questions.
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			The '''philosophy of perception''' is concerned with the nature of ] and the status of ], in particular how they relate to beliefs about, or knowledge of, the world.<ref name="Bonjour">cf. http://plato.stanford.edu/entries/perception-episprob/ BonJour, Laurence (2007): "Epistemological Problems of Perception." ''Stanford Encyclopedia of Philosophy'', accessed 1.9.2010.</ref> Any explicit account of perception requires a commitment to one of a variety of ] or ] views. Philosophers distinguish ] accounts, which assume that perceptions of objects, and ] or ]s about them, are aspects of an individual's mind, and ] accounts, which state that they constitute real aspects of the world external to the individual.<ref name="Bonjour" />{{Failed verification\|date=April 2020}} The position of ]—the 'everyday' impression of physical objects constituting what is perceived—is to some extent contradicted by the occurrence of perceptual illusions and hallucinations<ref>cf. http://plato.stanford.edu/entries/perception-problem/ Crane, Tim (2005): "The Problem of Perception." ''Stanford Encyclopedia of Philosophy'', accessed 1.9.2010; Drestske, Fred (1999): "Perception." In: Robert Audi, ''The Cambridge Dictionary of Philosophy'', Second Edition, Cambridge, Massachusetts: Cambridge University Press, pp. 654–658, here p. 656.</ref> and the relativity of perceptual experience<ref name="Bonjour" /> as well as certain insights in science.<ref>cf. ] (2006): Perception. In: Sahotra Sarkar/Jessica Pfeifer (Eds.), ''The Philosophy of Science: An Encyclopedia'', New York: Routledge, pp. 545–550, here p. 546 ff.</ref> ] conceptions include ] and ]. ] conceptions include ] and ].<ref name="Bonjour" /> Recent philosophical work have expanded on the philosophical features of perception by going beyond the single paradigm of vision (for instance, by investigating the uniqueness of olfaction<ref>{{cite book \|author1=Ann-Sophie Barwich \|author-link=Ann-Sophie Barwich \|title=Smellosophy: What the Nose tells the Mind \|date=2020 \|publisher=Harvard University Press \|isbn=9780674983694 \|pages=384 \|url=https://www.hup.harvard.edu/catalog.php?isbn=9780674983694}}</ref>).
	==Categories of Perception==
	We can categorise perception as ''internal'' or ''external''.
	* Internal perception (]) tells us what's going on in our bodies. We can sense where our limbs are, whether we're sitting or standing; we can also sense whether we are hungry, or tired, and so forth.
	* Sense perception (]), tells us about the world outside our bodies. Using our senses of sight, hearing, touch, smell, and taste, we discover colors, sounds, textures, etc. of the world at large.

			==Scientific accounts of perception==
	The philosophy of perception is mainly concerned with exteroception. When philosophers use the word ''perception'' they usually mean exteroception, and the word is used in that sense everywhere below.
	==Perception as a Cognitive Act==
	As is often necessary in ], some simple observations put the whole discussion in context.

			An object at some distance from an observer will reflect light in all directions, some of which will fall upon the corneae of the ], where it will be focussed upon each ], forming an image. The disparity between the electrical output of these two slightly different images is resolved either at the level of the ] nucleus or in a part of the ] called 'V1'. The resolved data is further processed in the visual cortex where some areas have specialised functions, for instance area V5 is involved in the modelling of motion and V4 in adding colour. The resulting single image that subjects report as their experience is called a 'percept'. Studies involving rapidly changing scenes show the percept derives from numerous processes that involve time delays.<ref>see Moutoussis and Zeki (1997)</ref> Recent ] studies <ref>{{Cite journal\|title=Brain decoding: Reading minds\|year=2013 \|doi=10.1038/502428a \|last1=Smith \|first1=Kerri \|journal=Nature \|volume=502 \|issue=7472 \|pages=428–430 \|pmid=24153277 \|s2cid=4452222 \|doi-access=free \|bibcode=2013Natur.502..428S }}</ref> show that dreams, imaginings and perceptions of things such as faces are accompanied by activity in many of the same areas of brain as are involved with physical sight. Imagery that originates from the senses and internally generated imagery may have a shared ] at higher levels of cortical processing.
	Some obvious features of perception spring from the nature of the mind in general. Perception is a ] ''process''. Instances of the use of perception are mental ''events'' because perception is something that occurs, and it occurs in the ]. I may look outside and see that it is raining. Seeing is a process; and when I looked outside and saw the rain, a ''perceptual'' mental event, occurred. Or we might say a ''visual'' mental event occurred, since the event was one of my sense of sight.

			] is analyzed in term of pressure waves sensed by the ] in the ear. Data from the eyes and ears is combined to form a 'bound' percept. The problem of how this is produced, known as the ].
	Philosophers often refer to perceptual mental events as ''acts of perception''. This implies that perception is not merely something that occurs or happens, but that it is something that we ''do'', sometimes actively. We open our eyes and actively examine things, listen for particular sounds, and so forth. So we can proactively choose to perceive things. But a good deal of the time we do not ''choose'' to direct our senses in one direction or another. For example, I am sure I just happened to notice that it was raining a few days ago; I didn't, as it were, actively ''choose'' to look out the window. Our senses may be distracted here and there, so then perception is passive and not active. Nonetheless philosophers will often refer to perceptual events as ''acts'', although that need not imply that we are always proactive when we perceive; obviously sometimes we are passive.

			Perception is analyzed as a ] in which ] is used to transfer information into the mind where it is related to other information. Some psychologists propose that this processing gives rise to particular mental states (]) whilst others envisage a direct path back into the external world in the form of action (radical ]). Behaviourists such as ] and ] have proposed that perception acts largely as a process between a stimulus and a response but have noted that ]'s "] of the brain" still seems to exist. "The objection to inner states is not that they do not exist, but that they are not relevant in a functional analysis".<ref>Skinner 1953</ref> This view, in which experience is thought to be an incidental by-product of information processing, is known as ].
	According to philosophers who use the word ''act'' to refer to perceptual events, acts always have ''objects''. When we ''act'', or ''do'' something, there is always something else ''on which'' we act, or something ''to which we do it''. Talk of acts requires talk of objects. So if a perceptual event is an act, it has an object, something that the act is directed upon. And of course that's certainly the case with at least ''some'' kinds of perception. For example when I see the rain, the object of my visual act is the rain. And of course more generally, whenever we see, or hear, or touch, it at the very least ''appears'' that there is ''something that'' we see, or hear, or touch. The things that we perceive, that perception is ''of'', are the ''objects'' of perception.

			Contrary to the behaviouralist approach to understanding the elements of cognitive processes, ] sought to understand their organization as a whole, studying perception as a process of ].
	==What are the immediate objects of perception?==
	The rest of this article addresses this question.
	===Immediacy===
	To understand this question, we must agree what ''immediate'' means.

			==Problem of Perception==
	Suppose you see the ] on ]. You aren't seeing the President ''immediately'', or directly. You're looking at a television screen. On that screen is a moving, colored shape that greatly resembles the President, but that shape is not the President ''himself''. It is only a picture that represents the President. Does that mean that you aren't seeing the President at all? Not in any sense? Of course not. You are seeing the President indirectly, by means of seeing a representation of him.
			Important philosophical problems derive from the ] of perception—how we can gain knowledge via perception—such as the question of the nature of ].<ref>Chalmers DJ. (1995) "Facing up to the hard problem of consciousness." ''Journal of Consciousness Studies'' 2, 3, 200–219</ref> Within the biological study of perception naive realism is unusable.<ref>Smythies J. (2003) "Space, time and consciousness." ''Journal of Consciousness Studies'' 10, 3, 47–64.</ref> However, outside biology modified forms of naive realism are defended. ], the eighteenth-century founder of the ], formulated the idea that sensation was composed of a set of data transfers but also declared that there is still a direct connection between perception and the world. This idea, called direct realism, has again become popular in recent years with the rise of ].

			The succession of data transfers involved in perception suggests that ] are somehow available to a perceiving subject that is the substrate of the percept. Indirect realism, the view held by ] and ], proposes that we can only be aware of ]s of objects. However, this may imply an infinite regress (a perceiver within a perceiver within a perceiver...), though a finite regress is perfectly possible.<ref>Edwards JC. (2008) "Are our spaces made of words?" ''Journal of Consciousness Studies'' 15, 1, 63–83.</ref> It also assumes that perception is entirely due to data transfer and information processing, an argument that can be avoided by proposing that the percept does not depend wholly upon the transfer and rearrangement of data. This still involves basic ontological issues of the sort raised by ]<ref>Woolhouse RS and Franks R. (1998) ''GW Leibniz, Philosophical Texts'', Oxford University Press.</ref> Locke, ], ] and others, which remain outstanding particularly in relation to the ], the question of how different perceptions (e.g. color and contour in vision) are "bound" to the same object when they are processed by separate areas of the brain.
	When you see the President on television, your visual act has two objects; the more direct or immediate object is a moving picture on the television screen; the indirect or mediate object is the President himself.

			Indirect realism (representational views) provides an account of issues such as perceptual contents,<ref>Siegel, S. (2011)."The Contents of Perception", ''The Stanford Encyclopedia of Philosophy'' (Winter 2011 Edition), Edward N. Zalta (ed.), URL = <http://plato.stanford.edu/archives/win2011/entries/perception-contents/>.</ref><ref>Siegel, S.: The Contents of Visual Experience. New York: Oxford University Press. 2010</ref> ], dreams, imaginings, ], illusions, the resolution of ], the resolution of ], the modelling of motion that allows us to watch TV, the sensations that result from direct brain stimulation, the update of the mental image by saccades of the eyes and the referral of events backwards in time. Direct realists must either argue that these experiences do not occur or else refuse to define them as perceptions.
	The immediate object of perception is anything we see first. By seeing it, we may be able to see other things with the same act of perception.

			Idealism holds that reality is limited to mental qualities while skepticism challenges our ability to know anything outside our minds. One of the most influential proponents of idealism was ] who maintained that everything was mind or dependent upon mind. Berkeley's idealism has two main strands, ] in which physical events are viewed as a special kind of mental event and ]. ] is probably the most influential proponent of skepticism.
	===Sense-data===
	The concept of ''sense-data'' is very influential and widely used in the theory of perception.

			A fourth theory of perception in opposition to naive realism, ], attempts to find a middle path between direct realist and indirect realist theories, positing that ] is a process of dynamic interplay between an organism's sensory-motor capabilities and the environment it brings forth.<ref>p 206, Varela F, Thompson E, Rosch E (1991) "The Embodied Mind: Cognitive Science and Human Experience" MIT Press</ref> Instead of seeing perception as a passive process determined entirely by the features of an independently existing world, enactivism suggests that organism and environment are structurally coupled and co-determining. The theory was first formalized by Francisco Varela, Evan Thompson, and Eleanor Rosch in "The Embodied Mind".<ref>Varela F, Thompson E, Rosch E (1991) "The Embodied Mind: Cognitive Science and Human Experience" MIT Press</ref>
	Many philosophers have said that the ''most immediate'' objects of perception are ''mental'' objects -- objects in the mind. So within the mind there are two different items; there is a perceptual ''act'', and there is a mental ''object'', which represents things outside of the mind.

			==Spatial representation==
	So for example, according to this widespread opinion of philosophers, when I see the President on TV the very first thing that I perceive is <em>an image of the President in my mind</em>, and this image represents the moving picture on the television screen, and that moving picture on the television screen in turn represents the President himself.
			An aspect of perception that is common to both realists and anti-realists is the idea of mental or ]. ] concluded that things appear extended because they have attributes of colour and solidity. A popular modern philosophical view is that the brain cannot contain images so our sense of space must be due to the actual space occupied by physical things. However, as René Descartes noticed, perceptual space has a projective geometry, things within it appear as if they are viewed from a point. The phenomenon of ] was closely studied by artists and architects in the Renaissance, who relied mainly on the 11th century polymath, ] (Ibn al-Haytham), who affirmed the visibility of perceptual space in geometric structuring projections.<ref>{{cite journal\|title=La perception de la profondeur: Alhazen, Berkeley et Merleau-Ponty\|author=Nader El-Bizri\|journal=Oriens-Occidens, CNRS\|volume=5\|year=2004\|publisher=]\|pages=171–184\|author-link=Nader El-Bizri}}</ref><ref>{{cite journal\|title=In Defence of the Sovereignty of Philosophy: al-Baghdadi's Critique of Ibn al-Haytham's Geometrisation of Place\|author=Nader El-Bizri\|journal=]\|volume=17\|year=2007\|publisher=]\|pages=57–80\|doi=10.1017/s0957423907000367\|s2cid=170960993 }}</ref> Mathematicians now know of many types of projective geometry such as complex ] that might describe the layout of things in perception (see Peters (2000)) and it has also emerged that parts of the brain contain patterns of electrical activity that correspond closely to the layout of the retinal image (this is known as ]). How or whether these become conscious experience is still unknown (see McGinn (1995)).

			==Beyond spatial representation==
	These immediate mental objects of perception have been called ''sense-data'', ''ideas'' or ''percepts''. So that mental image I am supposed to have of the President is called a visual ] (plural, ]) or a visual idea of the President, or a visual percept of the President. We have mental awareness of those Presidential sense-data -- not with our eyes, of course, because our eyes are in the physical world, and sense-data are in the mind. Those Presidential sense-data are caused by the image of the President on the TV screen. And the sense-data represent the President to us. So generally there are supposed to be mental, internal objects of perception, which represent physical, external objects. "Internal" here just means "inside the mind" (though of course you can guess that that phrase is open to different interpretations); and "external" means, correspondingly, "outside the mind" or "in the physical world."
			Traditionally, the philosophical investigation of perception has focused on the sense of vision as the paradigm of sensory perception.<ref>{{cite book \|last1= Barwich \|first1=Ann-Sophie \|date=2020 \|title=Smellosophy: What the Nose tells the Mind \|publisher=Harvard University Press \|page=310 \|isbn=9780674983694}}</ref> However, studies on the other sensory modalities, such as the sense of smell, can challenge what we consider characteristic or essential features of perception. Take olfaction as an example. Spatial representation relies on a "mapping" paradigm that maps the spatial structures of the stimuli onto discrete neural structures and representations.<ref>{{Cite web\|date=2020-12-11\|title=Nautilus {{!}} Science Connected\|url=http://nautil.us/\|access-date=2020-12-11\|website=Nautilus}}</ref> However, olfactory science has shown us that perception is also a matter of associative learning, observational refinement, and a decision-making process that is context-dependent. One of the consequences of these discoveries on the philosophy of perception is that common perceptual effects such as conceptual imagery turn more on the neural architecture and its development than the topology of the stimulus itself.<ref>{{cite book \|last1= Barwich \|first1=Ann-Sophie \|date=2020 \|title=Smellosophy: What the Nose tells the Mind \|publisher=Harvard University Press \|page=311 \|isbn=9780674983694}}</ref>

			==See also==
	===The argument from illusion===
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	Sense-data, internal objects of perception, might sound strange. Why think that such things even exist? Why think that we perceive internal objects at all? Why not think that we just immediately, or directly, perceive external objects? There is an argument, or rather several arguments, for this view. They go under one general title, <em>the argument from illusion</em>. The conclusion of the argument from illusion is: "The immediate objects of perception are sense-data." So what reasons can be given to support this conclusion?
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			==Notes==
	Consider several examples of what we might call "perceptual illusions." Take a perfectly straight stick, and submerge it in a pond: then the stick will <em>appear</em> to be bent. The bend in the stick isn't real -- it's illusory. Or you're driving across the countryside and you see some hills in the distance, which appear bluish. Then as you approach nearer to the hills you see that they are actually green. The bluish color the hills originally appeared to have wasn't real; it was illusory.
			{{Reflist}}

			==References==
	Suppose you want to say that the <em>immediate</em> or <em>direct</em> objects of your perception are the stick itself, and the hills themselves. Then it appears that you're saying that <em>the stick</em> was first straight, and then it was bent. You say it's <em>the stick</em> that is the direct object of your perception. And <em>something</em> did look straight, and then bent. So if the direct object of your perception was the stick, why then it must have been the <em>stick</em> that was first straight, and then bent. But that's ridiculous if we know nothing happened to change the stick.
			* {{cite SEP \|url-id=perception-episprob \|title=Epistemological Problems of Perception \|first=L. \|last=BonJour \|date=2007}}
			* {{cite SEP \|url-id=perception-problem \|title=The Problem of Perception \|first=Tim \|last=Crane \|first2=C. \|last2=French \|date=2015}}
			* {{cite IEP \|url-id=cog-pene \|title=Cognitive Penetrability of Perception and Epistemic Justification \|first=Christos \|last=Georgakakis \|first2=Luca \| last2= Moretti}}
			* {{cite SEP \|url-id=merleau-ponty \|title=Maurice Merleau-Ponty \|date=2004 \|last1=Flynn \|first1=Bernard}}
			* {{cite IEP \|url-id=epis-per \|title=Epistemology of Perception \|first=Daniel \|last=O'Brien}}
			* {{cite IEP \|url-id=perc-obj \|title=Objects of Perception \|first=Daniel \|last=O'Brien}}
			* {{cite SEP\| last=Siegel \|first=Susanna \|date=2005 \|title=The Contents of Perception\| url-id=perception-contents}}

			==Further reading==
	We could say, instead, that when you were looking at the stick, you had a series of stick sense-data; and the sense-data were at first straight, and then bent. The stick itself, all the while, was perfectly straight. In the same way, you want to say that, when you were looking at the hills, <em>something</em> looked bluish, and then it looked green; but it wasn't the <em>hills</em> that actually <em>were</em> bluish and then green; so we could say that you <em>immediately</em> perceived hill <em>sense</em>-<em>data</em>, and the sense-data were bluish, and then green. The hills, all the while, were the same color.

			===Historical===
	There are other similar cases.
			* Descartes, Rene (1641). ''Meditations on First Philosophy''.
			* Hume, David (1739–40). ''A Treatise of Human Nature: Being An Attempt to Introduce the Experimental Method of Reasoning Into Moral Subjects''.
			* Kant, Immanuel (1781). ''Critique of Pure Reason''. Norman Kemp Smith (trans.) with preface by ], Palgrave Macmillan.
			* Locke, John (1689). ''An Essay Concerning Human Understanding''.
			* Russell, Bertrand (1912). ''The Problems of Philosophy'', London: Williams and Norgate; New York: Henry Holt and Company.

			===Contemporary===
	Suppose I press an eyeball with one finger while I hold up another finger in front of my nose. Suddenly the finger in front of my nose appears becomes double. Well, not <em>actually</em> double; it's just that the finger <em>appears</em> double, or that there are two images of the finger before there was just one. Now there are two of <em>something</em> there, right? But not of a finger. I'm not seeing two fingers! So we could say that there are two <em>sense</em>-<em>data</em>, and each of those sense-data represents the same finger.
			{{Wikibooks\|Consciousness Studies}}
			* Burge, Tyler (1991). "Vision and Intentional Content," in E. LePore and R. Van Gulick (eds.) ''John Searle and his Critics'', Oxford: Blackwell.
			* Chalmers DJ. (1995) "Facing up to the hard problem of consciousness." ''Journal of Consciousness Studies'' 2, 3, 200–219.
			* Dretske, Fred (1981). ''Knowledge and the Flow of Information'', Oxford: Blackwell.
			* Evans, Gareth (1982). ''The Varieties of Reference'', Oxford: Clarendon Press.
			* McDowell, John, (1982). "Criteria, Defeasibility, and Knowledge," ''Proceedings of the British Academy'', pp. 455–79.
			* McDowell, John, (1994). ''Mind and World'', Cambridge, Massachusetts: Harvard University Press.
			* McGinn, Colin (1995). "Consciousness and Space," In ''Conscious Experience'', Thomas Metzinger (ed.), Imprint Academic.
			* Mead, George Herbert (1938). "Mediate Factors in Perception," Essay 8 in ''The Philosophy of the Act'', Charles W. Morris with John M. Brewster, Albert M. Dunham and David Miller (eds.), Chicago: University of Chicago, pp. 125–139.
			* Moutoussis, K. and Zeki, S. (1997). "A Direct Demonstration of Perceptual Asynchrony in Vision," ''Proceedings of the Royal Society of London'', Series B: Biological Sciences, 264, pp. 393–399.
			* Noe, Alva/Thompson, Evan T.: Vision and Mind: Selected Readings in the Philosophy of Perception, Cambridge: MIT Press, 2002.
			* Peacocke, Christopher (1983). ''Sense and Content'', Oxford: Oxford University Press.
			* Putnam, Hilary (1999). ''The Threefold Cord'', New York: Columbia University Press.
			* Shoemaker, Sydney (1990). "Qualities and Qualia: What's in the Mind?" ''Philosophy and Phenomenological Research'' 50, Supplement, pp. 109–31.
			* Tong, Frank (2003). "Primary Visual Cortex and Visual Awareness," Nature Reviews, ''Neuroscience'', Vol 4, 219.
			* Tye, Michael (2000). ''Consciousness, Color and Content'', Cambridge, Massachusetts: MIT Press.

			{{Epistemology}}
	Or take a coin out of your pocket and hold it up, and turn it over. If you look at it straight on the coin looks perfectly circular; but as you turn it over it appears to have an oval shape. Now as you turn the coin over, just consider: you <em>are</em> seeing two different shapes, namely a circle and an oval. But the <em>coin</em> has just one shape, right? A circle, so we think. So if you are seeing two different shapes, those must be the shapes of <em>sense</em>-<em>data</em>, and not the coin; and the sense-data <em>represent</em> the coin.
			{{philosophy of mind}}
			{{Authority control}}

			]
	Next consider a very different sort of case. Suppose Mary starts ] ]. She's out for a walk, and unfortunately suffers a nervous breakdown. It looks to her like there is a herd of pink elephants bellowing and charging right at her. Now let's suppose these hallucinated elephants are extremely vivid. They are just as vivid to Mary as real elephants would be to you and I. Then here's the point: Mary would say that she is <em>seeing</em> pink elephants; now, we would deny that, and say she's just <em>hallucinating</em>; but her <em>]</em>, or how things appear to her, is <em>just the same</em> as how things would appear to <em>us</em> if we were to see a herd of pink elephants bearing down on us. For convenience we could refer to the <em>experience</em>, in either case, as the perceiving of sense-data. And then we could say that Mary's sense-data just don't represent anything. In fact, we could say that when someone hallucinates, she is just perceiving certain sense-data, and those sense-data don't represent anything.
			]

			]
	These examples relate only to visual perception, to the sense of sight, but there are similar examples of such "illusions" for the other senses.
			]

			]
	In all of these cases we can see that it is very convenient, on first glance anyway, to suppose that we <em>immediately</em> perceive sense-data. Therefore, the immediate objects of perception are sense-data. This includes <em>all</em> sense-perception -- sense-data occur not just when illusions occur, but <em>whenever</em> we perceive the external world.
			]

	===Representationalism===
	Various theories about perception use the concept of sense-data (or ideas, or percepts, or they may use some other term). One theory that immediately suggests itself, after one considers the argument from illusion, is called <em>the representational theory of perception</em>, or simply <em>representationalism</em>. Here is a definition:

	:] is the doctrine that in any act of perception, the immediate (direct) object of perception is a sense-datum, and this sense-datum represents an external object, which is the mediate (indirect) object of perception.

	Two 17th century philosophers, the Frenchman ], and the Englishman ] most prominently advocated this theory. The term they used was not "sense-datum" but "]." We do not concern ourselves with any differences in meaning that these terms might have. When we use "idea" below, we mean just a sense-datum. "Idea" as used in the theory of perception is a technical term, meaning roughly the same thing as "sense-datum." But I'm going to go on talking about sense-data.

	Representationalism asserts that sense-data ''represent'' external objects -- physical objects, properties, and events. But this immediately raises a question: <em>How well</em> do sense-data represent external objects, properties, and events? At least sometimes, they do not represent them at all well. When, for example, the stick sense-datum is bent, it does not perfectly represent the stick, which is straight. And similarly with the other illusions -- to the point of poor Mary whose pink elephant sense-data do not represent anything at all.

	===Primary and Secondary Qualities===
	Any claim that sense-data are always right, that they always give us accurate information about what the world is like, would clearly be over-simplistic. Several good examples above show it is wrong. If there are sense-data at all, they definitely do not always accurately represent the world as it is. That's <em>at least</em> the case with perceptual illusions.

	Both ] and ] recognized this. Beside various perceptual illusions, they said that another category of sense-data also did not adequately represent the ]. Both Descartes and Locke drew a distinction between two kinds of properties they called ''primary qualities'' and ''secondary qualities''. They said that sense-data of <em>primary</em> qualities can accurately represent external qualities in things; but sense-data of <em>secondary</em> qualities <em>cannot</em> accurately represent external qualities in things.

	The distinction between primary and secondary qualities is a fairly common sense sort of distinction.

	:''Some'' qualities things appear to have, the <em>primary</em> qualities, <em>really are</em> in the things. ''Other'' qualities things appear to have, the <em>secondary</em> qualities, are <em>only</em> in our minds; they are only aspects of how we perceive the world.

	For example, there's a real difference between the actual temperature of a kettle of water, and how warm the water feels when I put my hand in it. The water might be only lukewarm; but if my hand is extremely cold, when I put it in the water, the water is going to feel hot. So for the sake of accuracy we should distinguish between the <em>actual temperature</em> of the water, which is a matter of how fast the water molecules are vibrating, and the <em>perceived warmth</em> of the water, which is not a quality of the water, but instead of <em>how we perceive</em> the water. So then we could say: actual temperature is a <em>primary quality</em> of the water, but perceived warmth is a <em>secondary quality</em> of the water.

	Here's another example. We can say that a physical body's ], or its shape, is a primary quality; but the <em>perceived weight</em>, or how heavy a body feels, is a secondary quality. The perceived weight of a thing depends on who is lifting it, how tired they are, and so on. But the <em>mass</em> of a body doesn't depend on who's lifting it.

	Tastes and smells are often given as good examples of secondary qualities. When you bite into a lemon, and say it's sour, do you mean that the <em>lemon itself</em> is sour, or only that in your perception of the lemon, there is this element of sourness? Sure, you can say that the lemon is ], and that is a primary quality. The lemon's acidity doesn't depend on who or what is tasting it. But whether the lemon is <em>sour</em>, or how sour it tastes, can indeed depend on who is tasting it. So acidity (pH) is a primary quality, but sourness is a secondary quality.

	We can have ideas or sense-data of both primary and secondary qualities. But, say Descartes and Locke, and many who came after them, sense-data of primary qualities can actually represent primary qualities; but sense-data of secondary qualities do not represent secondary qualities, because really there aren't any secondary qualities in the object at all. We might just say that "secondary quality" refers to exactly the same thing as "sense-datum of a secondary quality."

	Examples again will help you understand this.

	We can have an idea of the actual pH of a lemon, its degree of acidity; and that idea can perfectly well represent the lemon's acidity. But when we get a sense-datum of sourness, it's not like there is something properly called "sourness" <em>in the lemon</em>. Acidity is all that's in the lemon; the sourness is just how we <em>perceive</em> the lemon, on account of its acidity.

	In the same way, we can have an accurate idea of the mass of a box; but when we lift the box and call it "heavy," that heaviness is just in the sense-datum we have of the box. There isn't anything properly called "heaviness" in the box <em>itself</em>; heaviness just how the box feels to us. Mass is what the box itself has; heaviness is how we feel the box to be.

	===Critical Realism===
	According to Locke and Descartes, some sense-data, namely the sense-data of secondary qualities, do not represent anything in the external world, even if they are <em>caused</em> by external qualities (primary qualities). Thus it is natural to adopt a theory of <em>]</em>, which may be defined as follows:

	:''Critical realism'' is the theory that ''some'' of our sense-data (for example, those of primary qualities) can and do accurately represent external objects, properties, and events, while other of our sense-data (for example, those of secondary qualities and perceptual illusions) do not accurately represent any external objects, properties, and events.

	This ] has been held by some very reasonable philosophers. But it does, by its talk of sense-data and representation, depend on or presuppose the truth of representationalism. If critical realism is correct, then representationalism would have to be a correct theory of perception.

	So, ''how well'' do sense-data represent external objects, properties, and events? Critical realism says: well enough. Some of our sense-data don't represent any external qualities, but through our sense-data of primary qualities, we can still know what the external world is like.

	===Skepticism===
	Suppose a rather extreme ] asks: "But ''how do you know'' that your sense-data represent primary qualities? Prove it." If we start to answer, the skeptic may interrupt. So let's listen now to the skeptic's argument, which is due to that influential Scotsman, ]:
	:"There is no proof that your sense-data represent primary qualities in the external world. Such a proof would be impossible. You have already concluded that whenever we perceive, we <em>immediately</em> perceive sense-data. That means that the only way we can perceive any external world is ''through'' sense-data. Perception is our external-world-information-gathering-faculty, and we have no other. So perception is the only way we can learn about the external world.

	:"Now you think that you can prove that your sense-data represent the external world. But how <em>could</em> you prove that? You would need to have some <em>evidence</em> that your sense-data represent the world. But have you ever perceived the connection between your sense-data and the world? Obviously not! You say that the <em>only</em> way we can perceive any external world is <em>through</em> sense-data. So we obviously could not use sense-data to prove that sense-data represent the world well, poorly, or at all.

	:"And it's even worse than that. Now, you think that you must always <em>immediately</em> perceive sense-data whenever you perceive anything. But that means that you cannot perceive the external world directly, nor can you perceive the connection between sense-data and the world. So how on earth could you even prove that the external world <em>exists</em>? How do you know that there is a world there on the other side of the sense-data? If perception is like a window onto the world, then how do you know that there is a world on the other side of the window? You can't prove it; all you have are sense-data. So we might as well give up all claims to having knowledge of any external world at all!"

	This sort of skeptical challenge is very hard to meet. This article does not meet it or discuss it, because we are talking about theories of perception; and skepticism is not a theory of perception.

	===Phenomenalism===
	A common reaction among philosophers to the foregoing ] has been simply to deny that an external world exists.

	Philosophers who hear the skeptic's challenge -- "There's no reason to think an external world exists" -- reply, "Well, no, I guess there isn't any reason to think that an external world exists. All there is, is sense-data. Physical objects are bundles of sense-data. When I hold up my hand, and I see it, I'm not seeing something external to my mind; I'm seeing a series, a whole bundle, of hand sense-data, and ''there is no hand'' apart from those hand sense-data. That's what my hand is -- a bundle of sense-data." Such philosophers get around skepticism, not by replying to the skeptic and proving the existence of an external world, but instead by saying that there is no external world.

	This approach is called ], the view that physical events are nothing more than a special kind of mental event. But it doesn't have to be stated in terms of just events. More generally, it can be defined as follows:

	:''Phenomenalism'' is the view that physical objects, properties, events, etc. (whatever is physical) are reducible to mental objects, properties, events, etc.; hence, ultimately, only mental objects, properties, events, etc., exist.

	In particular, we may reduce talk of physical bodies to talk of ''bundles of sense-data''. This logically goes with the bundle theory of objects. And the philosopher who is most famous for advocating ''both'' the bundle theory of objects, ''and'' phenomenalism, is the 18th century Irish philosopher, ]. Berkeley's version is more commonly called "]".

	This article does not discuss ] or phenomenalism at any length. It may however be interesting to note just how far from common sense the doctrine of representationalism has led.

	====The solipsism argument against phenomenalism====
	This article describes just one objection to phenomenalism, which uses a ].

	Suppose the phenomenalist is correct. This is the assumption we intend to disprove.

	Phenomenalism means that, for me, the physical world is all just a construction out of my sense-data. Now suppose you and I are talking about philosophy. On the face of it, there's my body, and my mind associated with it, and there's your body, with your body associated with it. But we are assuming that phenomenalism is true. That means that when I see <em>your body</em>, I'm not seeing an irreducibly physical body; I am seeing a bundle of sense-data in my own mind. Let's suppose I hear you saying all sorts of intelligent things, which I want to take as evidence of the existence of your mind, of what you are thinking. All those intelligent things you say are, after all, <em>sense-data in my own mind</em>. So I have no reason to think that either your body, or your mind exist. The phenomenalist has no reason to believe that any other minds, besides his own, exist.

	Why should the phenomenalist be surprised when we say that? After all, phenomenalism denies that an external world exists. Just remember what "external world" means according to the phenomenalist: it means the world <em>outside his own mind</em>. But that means that all other bodies <em>and other minds</em> are part of the external world. And so the phenomenalist is forced to an absurd conclusion. The argument in brief is:

	# If phenomenalism is true, then nothing that is thought to be in the external world exists.
	# But other minds besides my own are thought to be in the external world (since the external world is anything outside my own mind).
	# Therefore, other minds do not exist.

	The phenomenalist ends up having to believe ], which is the view that one's own mind is the only thing that exists -- that one is entirely alone in a universe that is completely in one's own mind. This is an absurd conclusion.

	Phenomenalism would force us to accept solipsism, so we had better reject our assumption that the phenomenalist is correct.

	There may be more to say about phenomenalism, but this article presents nothing further on the subject.

	===Direct Realism===
	Scottish philosopher ] lived at the same time as Hume. Reid argued strenuously against the notion that ideas, or sense-data, are the immediate objects of perception at all -- he rejected representationalism.

	One of Reid's arguments was very simple, and went like this: If representationalism is correct, then we are forced to either skepticism or phenomenalism. But skepticism and phenomenalism are both absurd; there surely is an external world, and we surely do have knowledge of it. So, by reductio ad absurdum, we must reject any theory that would force us to accept either skepticism or phenomenalism. So, we must reject ].

	What would it mean to reject representationalism? It would mean accepting that we do not perceive sense-data at all. When I look at my hand, I do not immediately perceive a bundle or series of hand sense-data, which represent my actual hand. No, I immediately perceive ''my hand''. I do not perceive any hand sense-data at all. So the view up for consideration now is that we immediately, directly perceive the external world.

	This view is called direct realism, which Reid championed brilliantly:

	:] is the view that the immediate (direct) objects of perception are external objects, qualities, and events.

	Do not confuse direct realism with the more naive view discussed earlier, that the world is exactly as we perceive it to be. Obviously, sometimes we misperceive the world. The direct realist does not deny that there are perceptual illusions. The claim is, rather, simply that when we do perceive something, what we directly perceive, the immediate <em>object</em> of perception, is in the external world, not in the mind.

	Nonetheless, the argument from illusion can be taken as an argument against direct realism because the argument from illusion shows the need to posit sense-data as the immediate objects of perception. How might direct realism answer the argument from illusion?

	One strategy is to show how all those different cases of misperception, failed perception, and perceptual relativity -- all those hard cases -- do not really make it necessary to suppose that there are sense-data. Those cases might be explained without having to talk about sense-data.

	Take first the case of the stick that looks bent in the water. Direct realism doesn't say that the stick actually is bent; it says, rather, that the stick, which is straight, can, in some unusual circumstances, look bent. And to say that it looks bent is just to say that the light, which is reflected from the stick, arrives at our eyes in a crooked pattern. So the stick can have more than one <em>appearance</em>. But the appearance of a stick isn't a <em>sense-datum in my mind</em>. It's a pattern of ], the sort of things that ] can study, that arrives at my ]. What's mysterious about that? A similar sort of thing can be said about the bluish color of the hills in the distance. Hills, and everything else, can appear with all sorts of different colors; but the color is simply the wavelength of light as it reaches my eye. If the light from the green hills has to traverse many miles, then it may be bluish when it arrives at my eyes. There's no need to suppose I am seeing bluish sense-data: nope, what I'm seeing is bluish light, which comes from the hills. The hills would reflect green light to my eyes if I were closer to them.

	Now the case of pressing on my eyeball, and getting a double image. Well, it's undeniable that, when I cross my eyes and seem to see two fingers, there are two of <em>something</em>. But of what? Why say there are two <em>sense</em>-<em>data</em>? Why not, instead, say that I have two <em>eyes</em>, and each eye gives me a different view upon the world. Usually the eyes are focused in the same direction; but sometimes they're not. And as a result, each eye sees things in a different way. That doesn't mean that I see two visual sense-data in my mind; but it does mean that there are two slightly different <em>acts of vision</em> going on. One for each eye! What's mysterious about that? Nothing, as far as I can tell. And similar things can be said about the coin that appears both circular and oval-shaped: so the same coin can reflect different patterns of light to my eye. Does that mean that I perceive two different sense-data? No, all it means is that I perceive the same coin in two different ways.

	Now as for Mary's vivid hallucination of the pink elephants. It was so vivid that the elephants were just as real as real elephants. We said this was evidence for thinking that she is perceiving sense-data; she sure as heck isn't perceiving elephants, and yet she seems to be hallucinating <em>something</em>. So maybe it's elephant sense-data that she is hallucinating. Well, that seems like a pretty tough case to deal with. It definitely does seem that there is an object, in <em>some</em> sense, of Mary's hallucination; but this object is only in her mind. Isn't that what we'd call sense-data?

	The direct realist might reply to that case as follows: Mary was not perceiving anything at all; she was hallucinating. That's a different, though related, mental process. So maybe Mary has visual images of some sort when she is hallucinating; that <em>wouldn't</em> mean that she has such images when she engages in actual sense-perception.

	This may not be a particularly strong reply. If there are visual images when we hallucinate, it seems reasonable to think that there are visual images when we see. It's the same way with dreams: if there are visual and auditory images of some sort in our minds, when we dream, it seems reasonable to think that there are visual and auditory images, or sense-data, when we are awake and perceiving things. We might ask for a better reply.

	Some people end up denying that there are any such things as mental images at all, but this is rather hard to maintain, since we seem to be able to imagine all sorts of things: for example, here's something that will give you an image: imagine a square, then imagine the top of the ] popping off and disappearing, and the two sides of the square collapsing together at a point, to make a ]. Even if it should happen that perception does not involve images, <em>other</em> mental processes, like ], certainly <em>seem</em> to.

	===Closing note on mental images===
	The topic of ] is very complicated and controversial, but I will tell you my own view about it, which is again similar to Reid's, just to bring this discussion to some closure. My own view is that in fact, in <em>some</em> sense, we <em>do indeed</em> have images of various sorts in our minds when we perceive, and dream, and hallucinate, and use our imaginations. But when we actually perceive things, in no sense whatsoever can our sensory images, or sensations, if you will (that's Reid's word), be considered the <em>objects</em> of perception, or attention, at all. The only objects of perception are external objects. Even if perception is accompanied by images, or sensations, it's wrong to say we <em>perceive</em> sensations.

	== See also ==
	* ]
	* ]
	* ]

Latest revision as of 17:33, 21 November 2024

Branch of philosophy

The philosophy of perception is concerned with the nature of perceptual experience and the status of perceptual data, in particular how they relate to beliefs about, or knowledge of, the world. Any explicit account of perception requires a commitment to one of a variety of ontological or metaphysical views. Philosophers distinguish internalist accounts, which assume that perceptions of objects, and knowledge or beliefs about them, are aspects of an individual's mind, and externalist accounts, which state that they constitute real aspects of the world external to the individual. The position of naïve realism—the 'everyday' impression of physical objects constituting what is perceived—is to some extent contradicted by the occurrence of perceptual illusions and hallucinations and the relativity of perceptual experience as well as certain insights in science. Realist conceptions include phenomenalism and direct and indirect realism. Anti-realist conceptions include idealism and skepticism. Recent philosophical work have expanded on the philosophical features of perception by going beyond the single paradigm of vision (for instance, by investigating the uniqueness of olfaction).

Scientific accounts of perception

An object at some distance from an observer will reflect light in all directions, some of which will fall upon the corneae of the eyes, where it will be focussed upon each retina, forming an image. The disparity between the electrical output of these two slightly different images is resolved either at the level of the lateral geniculate nucleus or in a part of the visual cortex called 'V1'. The resolved data is further processed in the visual cortex where some areas have specialised functions, for instance area V5 is involved in the modelling of motion and V4 in adding colour. The resulting single image that subjects report as their experience is called a 'percept'. Studies involving rapidly changing scenes show the percept derives from numerous processes that involve time delays. Recent fMRI studies show that dreams, imaginings and perceptions of things such as faces are accompanied by activity in many of the same areas of brain as are involved with physical sight. Imagery that originates from the senses and internally generated imagery may have a shared ontology at higher levels of cortical processing.

Sound is analyzed in term of pressure waves sensed by the cochlea in the ear. Data from the eyes and ears is combined to form a 'bound' percept. The problem of how this is produced, known as the binding problem.

Perception is analyzed as a cognitive process in which information processing is used to transfer information into the mind where it is related to other information. Some psychologists propose that this processing gives rise to particular mental states (cognitivism) whilst others envisage a direct path back into the external world in the form of action (radical behaviourism). Behaviourists such as John B. Watson and B.F. Skinner have proposed that perception acts largely as a process between a stimulus and a response but have noted that Gilbert Ryle's "ghost in the machine of the brain" still seems to exist. "The objection to inner states is not that they do not exist, but that they are not relevant in a functional analysis". This view, in which experience is thought to be an incidental by-product of information processing, is known as epiphenomenalism.

Contrary to the behaviouralist approach to understanding the elements of cognitive processes, gestalt psychology sought to understand their organization as a whole, studying perception as a process of figure and ground.

Problem of Perception

Important philosophical problems derive from the epistemology of perception—how we can gain knowledge via perception—such as the question of the nature of qualia. Within the biological study of perception naive realism is unusable. However, outside biology modified forms of naive realism are defended. Thomas Reid, the eighteenth-century founder of the Scottish School of Common Sense, formulated the idea that sensation was composed of a set of data transfers but also declared that there is still a direct connection between perception and the world. This idea, called direct realism, has again become popular in recent years with the rise of postmodernism.

The succession of data transfers involved in perception suggests that sense data are somehow available to a perceiving subject that is the substrate of the percept. Indirect realism, the view held by John Locke and Nicolas Malebranche, proposes that we can only be aware of mental representations of objects. However, this may imply an infinite regress (a perceiver within a perceiver within a perceiver...), though a finite regress is perfectly possible. It also assumes that perception is entirely due to data transfer and information processing, an argument that can be avoided by proposing that the percept does not depend wholly upon the transfer and rearrangement of data. This still involves basic ontological issues of the sort raised by Leibniz Locke, Hume, Whitehead and others, which remain outstanding particularly in relation to the binding problem, the question of how different perceptions (e.g. color and contour in vision) are "bound" to the same object when they are processed by separate areas of the brain.

Indirect realism (representational views) provides an account of issues such as perceptual contents, qualia, dreams, imaginings, hallucinations, illusions, the resolution of binocular rivalry, the resolution of multistable perception, the modelling of motion that allows us to watch TV, the sensations that result from direct brain stimulation, the update of the mental image by saccades of the eyes and the referral of events backwards in time. Direct realists must either argue that these experiences do not occur or else refuse to define them as perceptions.

Idealism holds that reality is limited to mental qualities while skepticism challenges our ability to know anything outside our minds. One of the most influential proponents of idealism was George Berkeley who maintained that everything was mind or dependent upon mind. Berkeley's idealism has two main strands, phenomenalism in which physical events are viewed as a special kind of mental event and subjective idealism. David Hume is probably the most influential proponent of skepticism.

A fourth theory of perception in opposition to naive realism, enactivism, attempts to find a middle path between direct realist and indirect realist theories, positing that cognition is a process of dynamic interplay between an organism's sensory-motor capabilities and the environment it brings forth. Instead of seeing perception as a passive process determined entirely by the features of an independently existing world, enactivism suggests that organism and environment are structurally coupled and co-determining. The theory was first formalized by Francisco Varela, Evan Thompson, and Eleanor Rosch in "The Embodied Mind".

Spatial representation

An aspect of perception that is common to both realists and anti-realists is the idea of mental or perceptual space. David Hume concluded that things appear extended because they have attributes of colour and solidity. A popular modern philosophical view is that the brain cannot contain images so our sense of space must be due to the actual space occupied by physical things. However, as René Descartes noticed, perceptual space has a projective geometry, things within it appear as if they are viewed from a point. The phenomenon of perspective was closely studied by artists and architects in the Renaissance, who relied mainly on the 11th century polymath, Alhazen (Ibn al-Haytham), who affirmed the visibility of perceptual space in geometric structuring projections. Mathematicians now know of many types of projective geometry such as complex Minkowski space that might describe the layout of things in perception (see Peters (2000)) and it has also emerged that parts of the brain contain patterns of electrical activity that correspond closely to the layout of the retinal image (this is known as retinotopy). How or whether these become conscious experience is still unknown (see McGinn (1995)).

Beyond spatial representation

Traditionally, the philosophical investigation of perception has focused on the sense of vision as the paradigm of sensory perception. However, studies on the other sensory modalities, such as the sense of smell, can challenge what we consider characteristic or essential features of perception. Take olfaction as an example. Spatial representation relies on a "mapping" paradigm that maps the spatial structures of the stimuli onto discrete neural structures and representations. However, olfactory science has shown us that perception is also a matter of associative learning, observational refinement, and a decision-making process that is context-dependent. One of the consequences of these discoveries on the philosophy of perception is that common perceptual effects such as conceptual imagery turn more on the neural architecture and its development than the topology of the stimulus itself.

Notes

^ cf. http://plato.stanford.edu/entries/perception-episprob/ BonJour, Laurence (2007): "Epistemological Problems of Perception." Stanford Encyclopedia of Philosophy, accessed 1.9.2010.
cf. http://plato.stanford.edu/entries/perception-problem/ Crane, Tim (2005): "The Problem of Perception." Stanford Encyclopedia of Philosophy, accessed 1.9.2010; Drestske, Fred (1999): "Perception." In: Robert Audi, The Cambridge Dictionary of Philosophy, Second Edition, Cambridge, Massachusetts: Cambridge University Press, pp. 654–658, here p. 656.
cf. Alva Noë (2006): Perception. In: Sahotra Sarkar/Jessica Pfeifer (Eds.), The Philosophy of Science: An Encyclopedia, New York: Routledge, pp. 545–550, here p. 546 ff.
Ann-Sophie Barwich (2020). Smellosophy: What the Nose tells the Mind. Harvard University Press. p. 384. ISBN 9780674983694.
see Moutoussis and Zeki (1997)
Smith, Kerri (2013). "Brain decoding: Reading minds". Nature. 502 (7472): 428–430. Bibcode:2013Natur.502..428S. doi:10.1038/502428a. PMID 24153277. S2CID 4452222.
Skinner 1953
Chalmers DJ. (1995) "Facing up to the hard problem of consciousness." Journal of Consciousness Studies 2, 3, 200–219
Smythies J. (2003) "Space, time and consciousness." Journal of Consciousness Studies 10, 3, 47–64.
Edwards JC. (2008) "Are our spaces made of words?" Journal of Consciousness Studies 15, 1, 63–83.
Woolhouse RS and Franks R. (1998) GW Leibniz, Philosophical Texts, Oxford University Press.
Siegel, S. (2011)."The Contents of Perception", The Stanford Encyclopedia of Philosophy (Winter 2011 Edition), Edward N. Zalta (ed.), URL = <http://plato.stanford.edu/archives/win2011/entries/perception-contents/>.
Siegel, S.: The Contents of Visual Experience. New York: Oxford University Press. 2010
p 206, Varela F, Thompson E, Rosch E (1991) "The Embodied Mind: Cognitive Science and Human Experience" MIT Press
Varela F, Thompson E, Rosch E (1991) "The Embodied Mind: Cognitive Science and Human Experience" MIT Press
Nader El-Bizri (2004). "La perception de la profondeur: Alhazen, Berkeley et Merleau-Ponty". Oriens-Occidens, CNRS. 5. Centre National de la Recherche Scientifique: 171–184.
Nader El-Bizri (2007). "In Defence of the Sovereignty of Philosophy: al-Baghdadi's Critique of Ibn al-Haytham's Geometrisation of Place". Arabic Sciences and Philosophy. 17. Cambridge University Press: 57–80. doi:10.1017/s0957423907000367. S2CID 170960993.
Barwich, Ann-Sophie (2020). Smellosophy: What the Nose tells the Mind. Harvard University Press. p. 310. ISBN 9780674983694.
"Nautilus | Science Connected". Nautilus. 2020-12-11. Retrieved 2020-12-11.
Barwich, Ann-Sophie (2020). Smellosophy: What the Nose tells the Mind. Harvard University Press. p. 311. ISBN 9780674983694.

References

BonJour, L. (2007). "Epistemological Problems of Perception". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy.
Crane, Tim; French, C. (2015). "The Problem of Perception". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy.
Georgakakis, Christos; Moretti, Luca. "Cognitive Penetrability of Perception and Epistemic Justification". Internet Encyclopedia of Philosophy.
Zalta, Edward N., ed. (2004). "Maurice Merleau-Ponty". Stanford Encyclopedia of Philosophy.
O'Brien, Daniel. "Epistemology of Perception". Internet Encyclopedia of Philosophy.
O'Brien, Daniel. "Objects of Perception". Internet Encyclopedia of Philosophy.
Siegel, Susanna (2005). "The Contents of Perception". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy.

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Latest revision as of 17:33, 21 November 2024

Scientific accounts of perception

Problem of Perception

Spatial representation

Beyond spatial representation

See also

Notes

References

Further reading

Historical

Contemporary