Perception, Contemporary Views

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Philosophical accounts of perception aim to give a coherent and systematic account of the nature of our sensory experiences. Philosophical accounts differ from scientific ones, which aim at explaining how the specific mechanisms of perception work. Philosophers are interested in general features that are common to anything that we might reasonably call perception, abstracting away from the specific mechanisms by which we perceive the world. Contemporary theorists of perception have proposed theories aimed at addressing a number of questions about perception, including the following: What accounts for the distinctive feel of our sensory experiences? Is perception a representational state with specific content (like desires and beliefs)? Is perception a "direct" awareness of the world? How does perception make possible beliefs and thoughts about the world? How do perceptions serve as reasons for belief, making possible knowledge of the world?

Appearance, Reality, and Phenomenal Character

One main source of philosophical puzzlement that has persisted since ancient times is the distinction between appearance and reality. To see the distinction, consider an example in which you see a ripe tomato sitting on a well-lit table. Assuming your eyesight is good, the tomato will appear a certain way to you; for example, it may appear red and round. This is a case of what we will call veridical perception. The tomato appears red and round to you, and in reality it is that way. It is, of course, also possible to misperceive, in which case the way things appear will not match the way they are. For example, if the tomato is in unusual lighting, it might appear to be purple rather than red. Likewise, if you are wearing shape-distorting glasses, the tomato might appear to be tall and skinny rather than short and plump, as it really is. These are cases of illusion, which involve objects appearing to you to have properties other than the ones that they have in reality. A second kind of misperception, distinct from illusion, is hallucination. Hallucinations are experiences in which it appears to you as if an object with certain properties is present, when in reality you are not in perceptual contact with any such object. For example, it might appear to you as if there is a red and round tomato before you when in fact there is no object there at all.

One problem that the possibility of misperception raises is epistemic and has to do with whether we are able to know things about the world. Skeptics about knowledge of the external world have held that, in order for you to have knowledge of the world, you must be able to rule out the possibility that you are now misperceiving. But, these skeptics claim, there are certain possibilities of radical misperception that you cannot properly rule outfor example, you can't rule out the possibility that you are right now dreaming, or the possibility that you are really a brain in a vat being fed experiences as of the external world by an evil superscientist who is directly stimulating your brain. Defenders of the common sense idea that we have perceptual knowledge attempt to reply to the skeptic's challenge.

As we will see, the possibility of misperception also provides a challenge for metaphysical accounts of the nature of perceptual appearances. The challenge arises in part because giving a theory of the nature of appearances requires accounting for what is sometimes called the phenomenal character of experience, or, more simply, the phenomenology of experience. The phenomenal character of a cognitive episode is, in Thomas Nagel's famous phrase, "what it is like" to undergo it. A feature unique to conscious states is that there is something it is like to be in them. There is, for example, a way it is like for one to see a tomato.

The phenomenal character of perceptual experiences seems to be a crucial part of what distinguishes such experiences from other conscious mental events such as occurrent thoughts, desires, and beliefs. For example, what it is like for you to think about a tomato that is in front of you with your eyes closed will be very different from what it is like to open your eyes and see the tomato. Seeing a tomato has a sensory, visual phenomenology that merely thinking about a tomato lacks. Although nonperceptual mental states like beliefs and desires arguably have a phenomenal character (for example, there is presumably something it is like for you to think about mathematical sums while in a sensory-deprivation tank with no perceptual experience at all), the phenomenal character of perceptual awareness is distinctive.

It may be that not all perceptions are conscious and so have a phenomenal character. It is common in psychology to distinguish between unconscious and conscious perceptions, and there is a growing psychological literature suggesting that much of the perceptual information that guides our actions is not conscious. (A good introduction to the psychological evidence is in Melvin Goodale's Sight Unseen.) It is a question of considerable philosophical interest what unconscious perception is and how to distinguish it from conscious perception. Nevertheless, we will focus here on theories of perception that seek to give an account of conscious perceptual experience.

There are several aspects of the phenomenal character of perceptual experience that philosophers have thought need to be reflected in a philosophical account of perception. First, there are differences in phenomenal character at the level of the different sense modalities. For example, what it is like to see a tomato is different from what it is like to taste, touch, or smell it. Each mode of perceptual awarenessvision, taste, touch, smell, and auditionhas its own distinctive sensory phenomenal character.

Second, there are similarities and differences in phenomenal character at the level of experiences within a sensory modality. For example, a tomato might appear to be red, another might appear to be green, and a third might appear to be very similar in color to the red one. Philosophers are also interested in the way that experiences from different perspectives give rise to differences in phenomenal character, even when there is no change in the way objects appear to be. For example, looking at the tomato from different angles or from nearer or farther away yields differences in the appearances, even though all of these experiences are arguably veridical perceptions and there is no change in the way the object appears to be. When viewed from close up, the tomato in a meaningful sense "appears larger" than when one looks at it from afar. Or, to take another example, it may be that part of the surface of a tomato "appears white" owing to the way the light is reflecting off its surface, even though in another sense the tomato appears uniformly red. These observations suggest a distinction between what we will call constant and perspectival modes of appearance talk. In the constant mode, saying that "an object appears so-and-so" implies that if you are not subject to an illusion, then the object is so-and-so. But this is not the case in the perspectival mode; it can be the case that "an object appears so-and-so" and that you are not subject to an illusion, while not being the case that the object really is so-and-so. For example, when you see the highlight on the tomato, it is correct to say that patch of the tomato "appears white" in the perspectival sense of appearance talk, but also correct to say that it "appears (to be) red" in the constant mode.

Since what we want in an account of perceptual experience is an account of perceptual consciousness, a correct theory of sense perception must be phenomenally adequate; it must do justice to the phenomenal character of experience.

Arguments from Illusion

A problem that divides philosophers of perception is how to account for the phenomenal character of experience while at the same time explaining the possibility of misperception. To see how the problem arises, consider a theory of perceptual experience that some philosophers have dubbed Naive Realism. As its name suggests, Naive Realism tries to take what is seen as a prereflective account of perception and use it as a philosophical theory of perception. According to them, perceptual consciousness is, in its fundamental nature, a relation of direct awareness between a perceiver and public objects and their properties. Moreover, it is these properties and objects of which one is aware that explain the phenomenal character of experience. Consider again our case of a tomato's visually appearing to you to be red and round. What explains the phenomenology of such an experience? The Naive Realist thinks that common sense is clear about what explains this: It is the tomato itself and the qualities of it presented to awareness that constitute what it is like to see the tomato. To explain what it is like to have a perceptual experience, we simply need to describe the objects that appear to you and their properties of which you are aware.

One challenge for Naive Realism is to explain differences in the phenomenal character of appearances described in the perspectival mode. For example, we saw that in the perspectival mode it is correct to say that the tomato viewed from afar appears smaller than when viewed from close up. Yet in both viewings of the tomato, it seems reasonable to suppose that you veridically perceive the size of the tomato, a property of the tomato that does not change. (This is why it is correct to say in the constant mode that whether the tomato is viewed from up close or from afar it appears to be the same size, say, roughly the size of your closed fist.) It seems, then, that what explains the difference in phenomenal character of these two viewings is not a property of the tomato, as the Naive Realist supposes.

Perhaps an even more difficult problem for the Naive Realist arises from the possibility of misperception. When you see the ripe tomato and your experience is veridical, you are in a perceptual state that we can describe by saying that "it appears to you as if there is something red and round before you." But it seems entirely possible for the very same type of state described in this way (complete with its distinctive phenomenal character) to occur as part of illusory or hallucinatory experience. For example, if you were wearing shape- and color-distorting glasses, it might be that what is in reality a tall, oblong, purple thing looks to you just like a plump, red tomato. This illusory experience might have the same phenomenal character as your veridical perception of a red, round tomato. The problem for the Naive Realist is that it cannot be in this case that the real color and shape properties of the thing you are seeing are what explain what it is like to see the object. The thing you are seeing is tall and purple, whereas your experience is as of something red and round.

The possibility of hallucination raises an exactly similar problem for Naive Realists. Consider a case in which you have a hallucination of a tomato when there is not one anywhere nearby. To fill out the case a bit, we might imagine that a futuristic superscientist stimulates your visual cortex in just the same way that it is stimulated when you see a tomato and thereby produces in you an experience that is every bit as vivid as a veridical perception of a tomato. If this were the case, it obviously can't be true that what explains the phenomenal character of your experience is a direct awareness of a real tomato that is red and round. In the case as described, there is not even a tomato there!

Although we have been focusing on specific visual examples involving seeing tomatoes, there is nothing special about our choice of examples. For any veridical perception that we could describe as one in which "it perceptually appears to you as if such-and-such is the case," it seems possible for you to be in a state with the very same phenomenal character that is an illusion or a hallucination. The problem for the Naive Realist is that they don't have the resources to explain the phenomenal character of these states, since their account at best only explains the phenomenal character of veridical perceptions.

The considerations here are related to a family of arguments that were commonly referred to in the twentieth century as "the argument from illusion." As might be apparent from our discussion, we can actually distinguish among arguments from perspective, illusion, and hallucination, depending on which of these phenomena is under consideration. Further on we will consider how different theorists propose to answer these problems, including responses on behalf of those who want to defend Naive Realism from the objections.

Sense-Data Theory

One historically important answer to the problems of perspective, illusion, and hallucination is that of the Sense-Data Theory. The theory is not as commonly held among contemporary theorists as it was among philosophers in the early twentieth century (such as G. E. Moore, H. H. Price, and C. D. Broad), but it still has a few defenders today (for example, Howard Robinson). According to Sense-Data Theorists, perception involves an immediate awareness of mental "Sense-Data," which are taken to be objects such that awareness of them fully determines their existence and nature.

Its proponents offer the Sense-Data Theory as the best explanation of the perspectival character of appearances, and of the possibility of misperception. Consider again our example of seeing the tomato. We saw that one challenge to Naive Realists is to answer questions about the perspectival character of appearances like this one: Why is it that looking at a tomato up close results in an experience that can be described in the perspectival mode as one in which the tomato appears larger than it does when you are standing far away from it? The Sense-Data Theorist will answer that this is because in the former case you are aware of a sense datum that really is larger than the sense datum you are aware of when you look at the tomato from afar. An advocate of the arguments from illusion and hallucination against Naive Realist might also ask this question: How is it, then, that a state with a single phenomenal characterfor example, a state in which it seems to you as if there is a red, round tomato before youcould occur either in a veridical perception or in a hallucination or in an illusion? The Sense-Data Theorist's answer is that the veridical perception, illusion, and hallucination all involve your being directly aware of sense data with the same properties, for example, sense data that are red and round.

According to the Sense-Data Theory, one is aware of objects and properties in the world only indirectly, in virtue of a more direct awareness of sense data and their properties. One challenge for the Sense-Data Theorist is to explain how sense data must be related to the world in order for one to perceive the world (albeit indirectly). For example, a Sense-Data Theorist owes us an answer to the following question: What makes it the case when you veridically perceive a tomato that being directly aware of a red, round sense datum counts as perceiving the real-world tomato? One possible reply would be that in order to perceive the tomato, you must be aware of a sense datum that has properties that resemble (or are isomorphic to) the properties of the tomato in the world. But this cannot be quite right. You can perceive a tomato even when your experience is a radical illusion such that you misperceive all of the tomatoes properties.

For example, if you look at the tomato through shape and color distorting lenses, the sense datum of which you are aware will not match the tomato in any of its shape or color properties (for example, the sense datum might be purple and tall while the tomato is short and red). But it might still be true that you see the tomato, even though you misperceive its properties. A second reply on behalf of the Sense-Data Theorist might be that you see the tomato if and only if the tomato causes the sense data of which you are aware. This proposal faces the problem that there are many different causes of the sense datum that don't count as things that you see. For example, the image on your retina is one of the causes of your perceptual experience (and its properties even seem to resemble the qualities of the sense data of which you are aware). But you do not see the images on your retina. Only eye doctors who are looking inside your eyes see retinal images. It seems that an object must cause an experience in the "right way" in order for the subject to perceive the object. It is a difficult problem, though, to say what this right way is.

Sense-data theories have been subject to many other objections. Arguments from illusion to the existence of sense-data theories have been criticized on grounds that they illicitly rely on a general principle of the following form: If it appears to you as if something has a certain property, then you are aware of something that really does have that property. Relying on this claim is sometimes referred to as the "sense-datum fallacy." The assumption has been thought to lead to absurd conclusions, like the conclusion that when an antique vase appears ancient and cracked to me, there is a sense datum that really is ancient and cracked. However, this conclusion might be blocked by restricting the properties mentioned in the principle to perceptible properties, such as color and shape. Moreover, the arguments from illusion, hallucination, and perspective should perhaps best be thought of as inferences to the best explanation. On this way of construing the arguments, the Sense-Data Theorist claims that postulating sense data offers the best explanation of the possibility of phenomenally identical illusions and hallucinations, and offers the best account of the perspectival nature of experience.

Other common objections to Sense-Data Theory allege that the view leads to skepticism, setting up a problematic epistemic "veil of perception" between the world and us, or that sense data are not scientifically respectable because they do not seem to be the sorts of things that fit easily into a physical picture of the world. In recent years, perhaps the most common objection to sense-data theories arises from a point about the phenomenal character of experience. Philosophers such as Gilbert Harman (1990/1997) and Michael Tye (1995) have claimed that there is a tension between Sense-Data Theory and what is sometimes metaphorically referred to as the "transparent" or "diaphanous" nature of experience. The idea that experience is transparent is the idea that perception, and in particular visual perception, seems on the face of it to be a direct presentation of objects and properties as they are in themselves, and does not seem to involve an awareness of subjective properties and objects that represent objects in the world, as the Sense-Data Theory suggests. In perception, it is argued, we seem to be aware only of public properties and objects. For example, philosophers who think experience is transparent will say that when you see the tomato and reflect on your experience, the only properties that you will seem to be aware of are the public properties of the tomato. As rendered by the transparency metaphor, experience doesn't seem to be an opaque object that we know to be related in some way to the external world, as we might expect if the Sense-Data Theory were true; rather, experience seems "transparent," and the world and its properties (metaphorically speaking) shine through it.

Intentionalist Theories

Philosophers such as Gilbert Harman (1990/1997), Michael Tye (1995), and Fred Dretske (1995), have suggested that by treating experience as an intentional state we can account for the transparency of experience while agreeing with the Sense-Data Theorist that there is a common kind of state involved in veridical perception that could also occur in illusion or hallucination. Intentional states are those with representational contents that can be correct or incorrect. A familiar example is belief. To believe that there is a tomato on the table, for example, is to be in a state that has a representational contentnamely the content There is a tomato on the table. This content can be correct or incorrect depending on whether there is in fact a tomato on the table.

Intentionalists claim that experience is like belief in being a state that represents the world as being some way or other, and they hold that the representational content of experience fully explains its phenomenal character. (Sometimes this claim of Intentionalists is put in terms of what is called a "supervenience claim": phenomenal properties supervene on intentional content, i.e., there can be no change in phenomenal qualities without a change in the intentional content of experience.) When it appears to you as if there is a tomato before you, for instance, you are in a state that represents certain properties typical of tomatoes (for example, being round, red, and so on). According to Intentionalists, the way in which the world is represented explains the phenomenal character of the experience. Moreover, the same experience could occur in a misperception. The experience is correct if there really is a tomato with those properties before me. It is illusory if there is an object there, but it isn't red or round. The experience is hallucinatory if there is no object there at all.

Intentionalists accommodate the transparency of perceptual experience by claiming that, even though perceptual experience involves a state that represents, introspection is open only to the properties and objects represented by the experience, all of which are taken by Intentionalist theorists to be external properties and objects. The way objects are represented in perceptual experiences is consequently not like the way in which objects are represented when one looks at a photograph of them. When one looks at a photograph of one's grandmother, one is aware of some of the features on the film in virtue of which the photograph represents Grandma (for example, the colors and shapes on the surface of the film). Perceptual experience is more like conceptual thought, at least thoughts that do not employ mental imagery. When one thinks about one's grandmother (supposing one doesn't use a bit of mental imagery to do so), one is not aware of the properties in virtue of which one's thought is about one's grandmother. One is simply aware of the represented object, one's grandmother. Likewise, according to Intentionalists, when one sees one's grandmother, one is not aware of the properties in virtue of which one's experience is representing grandmother; one is only aware of what is representedGrandmother and her properties.

Some early versions of Intentionalism claimed that perception is not merely similar to belief, it is in fact a kind of belief. (This was, for example, David Armstrong's view in his book Perception and the Physical World [1961]) However, such a view faces serious objections. A noncontroversial way of showing that experiences are not beliefs is to note that experiences are not revisable in light of counterevidence in the way that beliefs are. For example, one might believe that one's current experience is illusory or hallucinatory. If one has good enough reason to believe this, one can fail to believe the evidence of one's senses, even though the perceptual experience, complete with its phenomenal character, will remain intact.

A related question that arises for those who hold that perception is not a kind of belief is whether experience is like belief insofar as it essentially involves a deployment of concepts. Some philosophers of perception have propounded Conceptualism, the view that every sensory element of perception involves an exercise of concepts by the perceiver. Conceptualism is often held on the ground that the only way that a state can serve as a reason for belief is if the state is conceptual through and through. Conceptualism is defended in this way by Bill Brewer (1999) and John McDowell (1994), although both argue for the position in the context of defending Disjunctivism (a view explained below) rather than Intentionalism. Some theorists object to Conceptualism on the grounds that animals or small children can perceive the world even though they lack concepts that would allow them to form beliefs about the world. Others object to Conceptualism on the grounds that the fine-grained phenomenal character of experience suggests that experience has "nonconceptual content." These philosophers suggest that the complexity and specificity of the properties and objects that you see in a single glance outstrip your conceptual capacity to form conceptual thoughts about these objects and properties.

Several potential objections to Intentionalist theories have been raised in the philosophical literature. One challenge for Intentionalists is the same as a challenge raised above for Sense-Data Theorists, namely to give an account as to how an object must be related to perceptual experience in order for the experience to be a perception of the object. It has seemed to most Intentionalists that the answer to this question involves an object's causing the experience in "the right sort of way." (For example, your experience as of a tomato must be caused in the right way by a tomato in order for you to see a tomato.) But it is difficult to say what this "right sort of way" is.

Quite a few philosophers have objected that Intentionalism lacks the resources to explain what is distinctively sensory about the phenomenal character of experiences. This general objection is pressed in a variety of ways. Some philosophers (such as Christopher Peacocke 2001) have challenged Intentionalists to provide an account of facts about appearances described in the perspectival mode, such as the way the tomato appears smaller when one moves further away from it. Other theorists attack the alleged transparency of experience by citing examples of what they claim are experiences that do not seem to be about public objects or properties. In some examples of perceptual experience, these philosophers claim, we seem to be aware of objects or properties that are essentially private and depend on our awareness of them. Proposed examples include experiences involving afterimages, double vision, blurred vision, and the "inner light show" that one experiences when one shuts one's eyes tightly.

Still other philosophers have objected that Intentionalists cannot explain the difference in phenomenal character between perception and other intentional states such as thinking. Earlier it was suggested that the phenomenal character of seeing a tomato is very different from merely thinking about the tomato. But both seem to be intentional states, and it seems that they might have the very same contentfor example, the content There is a red and round tomato on the table. A challenge for the Intentionalist is to explain the difference between these two states. Some Intentionalists have suggested that the difference can be explained because perceptual experience is nonconceptual and plays a distinctive role in relation to beliefs and desires. A related challenge for Intentionalists is to distinguish between the phenomenal character of experiences in different modalities. For example, one can both feel the roundness of a tomato and also see the roundness. These states both represent the same property, the roundness of the tomato, so the Intentionalist might seem to be committed to thinking that the phenomenal character is the same. But of course the phenomenal character of the states is quite different.

Those who find the foregoing objections to Intentionalism compelling might still hold on to the idea that perception is an intentional state and that the content of the state in part explains the phenomenal character of experience. They will hold, however, that something in addition to the intentional content is required in order to account for the distinctively sensory phenomenal character of experience. Some philosophers (for example, Timothy Crane 1992) have suggested that in order to explain the phenomenal character of experience fully, we need to appeal not only to intentional contents but also to modes of presentation of those contents. For example, to explain the phenomenal character of your seeing the tomato we need to mention not only that you are in a state with the content that there is a red tomato before you, but also that this content is presented visually, rather than, say, tactilely. Others, such as Ned Block, suggest that we need to appeal to nonintentional properties of experience, sometimes called "qualia" in order to fully account for the phenomenal character of experience. This alternative is, in fact, consistent with the Sense-Data Theory. It is possible to develop a view according to which the perception of the tomato has an intentional content (for example, the content There is something red and round before you ) that partly explains the phenomenal character of the experience, while also arguing for the need to postulate an awareness of a mental sense datum with certain properties in order to give a complete explanation of the phenomenal character of experience. (This seems to be a view held by Christopher Peacocke in his book Sense and Content, though he speaks of awareness of "visual fields" rather than sense data.)


In recent years there has been a resurgence of attempts to defend Naive Realism by giving what is called a disjunctive account of experiences. Disjunctivists challenge the claim that for any veridical perceptual state of a subject (seeing a ripe tomato, for example), an event of the very same kind, individuated by its phenomenal character, could occur in a misperception. As stated earlier, one can describe the state of seeing the tomato as one in which "it appears to you as if there is something red and round before you," and this state can occur either in veridical perception, illusion, or hallucination. According to Disjunctivists, the state that we describe in this way is not a unified kind. The most that can be said about it is that this it is either (1) a state in which you are veridically perceiving a red and round tomato (in which case you are directly aware of the tomato and its properties) or (2) a state in which you are having a hallucination or an illusion that is indistinguishable from a veridical perception as of a tomato.

One might complain that so far, this is no theory at all, but at best a promise of one. The theory does not tell us anything, for example, about the phenomenal character of hallucinatory experiences. Disjunctivists, one might think, owe us an account of the phenomenal character of the "bad" side of the disjunct that involves hallucinatory experience. Many Disjunctivists resist the call to give a robust account of the phenomenal character of hallucinatory experience. For instance, Michael Martin, in "The Limits of Self Awareness" 2004, gives a purely epistemic characterization of hallucination. According to him, the most that can be said about the nature of hallucination is that it is indistinguishable from a genuine perception. For example, in the case where an advanced neuroscientist stimulates your visual cortex in exactly the way it is stimulated when you veridically perceive a tomato, Martin will say that the most fundamental thing we can say to explain the nature of this state is that this is a state such that you can't know purely on the basis of the experience that it isn't a genuine perception of a tomato. Many theorists, though, will think that the obvious explanation as to why your hallucination of a tomato can't be distinguished from a veridical perception is that the hallucination has a phenomenal character of a kind that requires a substantive metaphysical explanationfor example, the sort of explanation that Sense-Data Theorists and Intentionalists give.

Other Disjunctivists have made some tentative proposals for what accounts for the phenomenal character of hallucinatory states. Harold Langsam (1997), for example, says that it is possible to develop a theory according to which it is the physical regions of space around the subject where the object appears to be that are the relata of hallucination, and William Alston (1999) has suggested in passing that hallucination may involve an awareness of mental images. Such theorists face what might seem to be embarrassing questions that challenge their theoretical disunity. Given that their account of hallucinatory states fully explains the phenomenal character of experience, why not apply that same explanation to the case of veridical perception? Isn't it explanatory profligacy to rely on a disjunctive account when a unified one is available?

In response, Disjunctivists might counter that the explanatory cost of having an ununified view is well worth paying because alternative accounts of perception are subject to fatal flaws. In fact, a typical strategy of Disjunctivists has been to try to show that alternative theories of perception face insurmountable difficulties, leading to skepticism or making it mysterious how it is possible to think about the external world, or failing to do justice to the phenomenal character of experience.

See also Alston, William P.; Armstrong, David M.; Broad, Charlie Dunbar; Dretske, Fred; Harman, Gilbert; Illusions; McDowell, John; Moore, George Edward; Nagel, Thomas; Realism; Sensa.


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