Philosophy of perception

The next major area we'll consider is the theory of perception. Perception is, of course, one of our most important mental processes. But you might ask why it is I'm focusing on just that process, as opposed to other extremely important mental processes,

The reason is that sense perception is essentially the way that our minds are connected to the world. If we couldn't perceive anything, then we wouldn't know anything except what's going on in our own minds. Perception is, as it were, our window onto the world. So it is important for us to know some basic facts about this "window onto the world." Hopefully, it allows us access to the world as it really is.

First I want you to draw a distinction between two kinds of perception. We can perceive what's going on in our bodies; for example, 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. Call that [[internal perception]], or if you want an impressive long word, it's proprioception. We can also perceive the world outside our bodies; using the senses of sight, hearing, touch, smell, and taste, we discover the colors, sounds, textures, and so forth, of the world. Call that sense-perception[?], or if you want the word corresponding to "proprioception," it is exteroception[?]. So internal perception is the feeling of what's going on in our own bodies, while sense-perception[?] is the perception, using the senses, of the objects and qualities of the world at large.

As it is studied by philosophers, the theory of perception is really mainly concerned with sense-perception. And so when philosophers use this word "perception" really what they are usually talking about is sense-perception. And for the rest of this chapter that's what I will mean.

One of the most obvious things that can be said about perception is something that you could infer from what we said last time about the mind in general. Namely, that perception is a cognitive[?] process or function, and that instances of the use of perception are mental events. Mental events, because perception is something that occurs, and it occurs in the mind. Say I look out my window at home as I did a few days ago and that I see that it is raining. Seeing itself is a process; and when I looked out the window and saw that it was raining, a mental event, a perceptual mental event, occurred. Or we might say a visual mental event occurred, since the event was one of vision, of my sense of sight.

But philosophers typically refer to perceptual mental events as acts. So they talk about "acts of perception." This implies that perception is something not merely that occurs or happens, but that it is something that we do, sometimes actively. We do, after all, open our eyes and actively examine things, listen for particular sounds, and so forth. So we can actively choose to perceive things. But a good deal of the time I suppose we do not choose to direct our senses in one direction or another. For example, I am sure that 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. Seems to me that a lot of the time our senses are distracted here and there, so in that regard, perception is passive and not active. Nonetheless philosophers will often refer to perceptual events as acts, and so I will refer to them as "acts" as well, although just keep in mind that when I call them "acts," I do not mean to imply that we are always active when we perceive. Because obviously sometimes we are passive when we perceive.

Now so far, as is usual in philosophy, we have begun by saying some obvious things about perception. But gradually these obvious philosophical discussions grade away into some less obvious points. So don't worry, we won't always be saying such obvious things; they are necessary to put the whole discussion in context though.

Anyway, according to the philosophers who use the word "act" to refer to perceptual events, acts always have objects. Think of it like this: when we act, or do something, there is always something else on which we act, or something to which we do it. In short, 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 falling outside, the object of my visual act is the rain falling. 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.

Now what I want to do is to introduce a problem. This problem will generate all the rest of the discussion that we will cover in our discussion of the theory of perception, so it's a very important problem. It can be stated as the following question: What are the immediate objects of perception? In order to understand this question, you'd have to know what "immediate" means. And to introduce that term I'll use a very ordinary example.

You can see the President on television. Suppose you're watching the evening news and you see the President giving a press conference. But notice something here -- you aren't seeing the President immediately, or directly. You're looking at a television screen; and on that television screen is a moving, colored shape that greatly resembles the President. But of course that moving, colored shape is not the President himself. It's just just a picture that represents the President. Now let me ask: Does all that mean that you aren't seeing the President at all? Not in any sense? Of course not. You are seeing the President indirectly, via the television. So we could put it like this: you see the President by means of seeing a picture of him on TV. You don't see him immediately, or directly; you see him only by means of seeing a representation of him. Really, 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.

So this should give you an idea of what I'm talking about when I use the words "immediate object of perception[?]." The immediate object of perception is anything we see first, and by seeing this we are able to see other things with the same act of perception. So the picture of the President is a more immediate object of perception, and in seeing this we are able to see the President -- all in the same act of perception. So one and the same act of visual perception can have two objects; and the first object represents the second one.

All right, now we're to the point where I can introduce an extremely influential and widespread view in the theory of perception. Here goes. 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: on the one hand, there is a perceptual act; and on the other hand, there is a mental object, which represents things outside of the mind. So for example, when I see the President on TV, then according to this widespread opinion of philosophers, the very first thing that I perceive is an image of the President in my mind, 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.

In the history of philosophy the immediate mental objects of perception have had lots of different names, but sense-data and ideas have been the most widespread; our text also uses the term percept. So that mental image I am supposed to have of the President is called a sense-datum[?] (plural, sense-data[?]) or a visual idea of the President, or a percept of the President. We as it were mentally gaze upon 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."

I suppose this might seem strange -- I mean, the very suggestion that there even exist 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?

Well, philosophers have an argument, or rather several arguments, for this view. So let's look at some of these arguments. They go under one general title, the argument from illusion. 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?

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 appear 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.

Suppose you want to say that the immediate or direct objects of your perception are the stick itself, and the hills themselves. Then consider what it seems you're saying: it appears that you're saying that that the stick was first straight, and then it was bent. After all, you say it's the stick that is the direct object of your perception. And something sure did look straight, and then bent. So if the direct object of your perception was the stick, why then it must have been the stick that was first straight, and then bent. But that's ridiculous! It's contradictory. So 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, something looked bluish, and then it looked green; but it wasn't the hills that actually were bluish and then green; so we could say that you immediately perceived hill sense-data, and the sense-data were bluish, and then green. The hills, all the while, were the same color (green, presumably).

Next consider some similar sorts of cases. 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 actually double; it's just that the finger appears double, or that there are two images of the finger before there was just one. Now there are two of something there, right? But not of a finger. I'm not seeing two fingers! So we could say that there are two sense-data, and each of those sense-data represents the same finger.

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 are seeing two different shapes, namely a circle and an oval. But the coin has just one shape, right? A circle, so we think. So if you are seeing two different shapes, those must be the shapes of sense-data, and not the coin; and the sense-data represent the coin.

Next consider a very different sort of case. Suppose Mary goes bonkers and starts hallucinating[?] pink elephants[?]. She's out strolling on the Oval, and unfortunately suffers a nervous breakdown (probably due to her philosophy class). By God it sure 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 seeing pink elephants; now, we would deny that, and say she's just hallucinating; but her experience[?], or how things appear to her, is just the same as how things would appear to us if we were to see a herd of pink elephants bearing down on us. For convenience we could refer to the experience, 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.

I have given examples only from visual perception, from the sense of sight, but I could easily find similar examples of "illusions," if you want to call them that, regarding the other senses. I doubt it's necessary to give more examples though; I think you probably get the idea. In all of these cases we can see that it is very convenient, on first glance anyway, to suppose that we immediately perceive sense-data. Therefore, the immediate objects of perception are sense-data. This includes all sense-perception -- sense-data occur not just when illusions occur, but whenever we perceive the external world.

Now, there are different theories about perception which talk about sense-data (or ideas, or percepts, or they may use some other term). Different theories of perception employ the general sense-datum concept. But one theory in particular most immediately suggests itself, after one considers the argument from illusion, is called the representational theory of perception, or simply representationalism. Here is a definition:

Representationalism[?] 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.

This theory was most prominently advocated by two famous 17th century philosophers, the Frenchman Rene Descartes, and the Englishman John Locke. The term they used was not "sense-datum" but instead "idea." We need not concern ourselves with the differences in meaning that these terms might have. Just keep in mind that when we use "idea" in the theory of perception, 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.

Now I want you to notice something about representationalism: it says that sense-data represent external objects -- physical objects, properties, and events. But this immediately raises a question: How well 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.

Now suppose that someone were to claim that sense-data are always right; they always give us accurate information about what the world is like. Well, such a doctrine would be really na\x{00EF}ve. I mean, we have already seen lots of good examples that show it is wrong. Clearly, if there are sense-data at all, they definitely do not always accurately represent the world as it is. That's at least the case with perceptual illusions.

Both Descartes and Locke recognized this. And in addition to various perceptual illusions, they said that there was another category of sense-data which did not adequately represent external world[?]. Both Descartes and Locke drew a distinction between two kinds of properties. Well, in this context, they're called "qualities," but that's just another name for "properties"; for the sake of being traditional we will call them "qualities" as well, but we might as well have called them "properties." Anyway, they drew a distinction between primary and secondary qualities. And they said that sense-data of primary qualities can accurately represent external qualities in things; but sense-data of secondary qualities cannot accurately represent external qualities in things.

So it will be fairly important for me to explain the distinction between primary and secondary qualities. Really, it's a fairly commonsense sort of distinction. Some of the qualities things appear to have, the primary qualities, really are in the things; other of the qualities things appear to have, the secondary qualities, are only in our minds, that is, they're 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 actual temperature of the water, which is a matter of how fast the water molecules are vibrating, and the perceived warmth of the water, which is not a quality of the water, but instead of how we perceive the water. So then we could say: actual temperature is a primary quality of the water, but perceived warmth is a secondary quality of the water.

Here's another example. We can say that a physical body's mass, or its shape, is a primary quality; but the perceived weight, 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 mass of a body doesn't depend on who's lifting it. Or take tastes and smells -- they 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 lemon itself 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 acidic, 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 sour, or how sour it tastes, can indeed depend on who is tasting it. So acidity is a primary quality, but sourness is a secondary quality. That's the distinction then between primary and secondary qualities -- OK?

Now back to the point. We can have ideas or sense-data of both primary and secondary qualities. But, say Descartes and Locke, and a lot of people 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 there in the thing at all. In fact, if you prefer, 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 see 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" in the lemon. Acidity is all that's in the lemon; the sourness is just how we perceive 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 itself; heaviness just how the box feels to us. Mass is what the box itself has; heaviness is how we feel the box to be.

So some of our sense-data, namely the sense-data of secondary qualities, do not represent anything in the external world, even if they are caused by external qualities (by primary qualities). Thus it is natural to adopt a theory that our text calls critical realism[?], which may be defined as follows:

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

This is, I think, a very reasonable theory. It 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. I mean that, if critical realism is correct, then representationalism would have to be a correct theory of perception.

Remember the question I asked: 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.

But now suppose a rather extreme skeptic comes up to us and asks: "But how do you know that your sense-data represent primary qualities? Prove it." And if we start to answer, the skeptic will interrupt. So let's listen now to the skeptic's argument, which is due to that influential Scotsman, David Hume: "Don't waste your breath trying to prove it. You can't prove that your sense-data represent primary qualities in the external world; such a proof would be impossible. Look at what you have already concluded. You have concluded that whenever we perceive, we immediately perceive sense-data. That means that the only way we can perceive any external world is through sense-data. Notice, we don't have any other way to get information about the external world. Perception is all we have; perception is, after all, the external-world-information-gathering-faculty. That's what perception is. 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 could you prove that? You would need to have some evidence 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 only way we can perceive any external world is through 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 immediately 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 exists? 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. And I do not propose to meet it or discuss it here, because we are talking about theories of perception; and skepticism is not a theory of perception. We will talk more about it at the appropriate time.

But what I do want to talk about is a surprisingly common reaction to the foregoing skepticism. Common historically among philosophers, not among ordinary people. One reaction to this skepticism has been, namely, simply to deny that an external world exists! They say that everything that exists is mental. As Dave Berry says, I am not making this up! It's completely true! There is a long and surprisingly respectable tradition of philosophers who hear the skeptic's challenge -- "There's no reason to think an external world exists" -- and then 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!" So these philosophers manage get around skepticism; but not by replying to the skeptic and proving the existence of an external world, but instead by saying that there is no external world.

But we have encountered this wacky theory before -- just last time, in fact. It is called "phenomenalism[?]," which we defined, as you may recall, as follows: "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.

So 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, George Berkeley. Berkeley's version is more commonly called "subjective idealism[?]"; "phenomenalism" is a more recently coined term.

I don't propose to discuss Berkeley or phenomenalism at any length. I simply want you to note just how far away from common sense this doctrine of representationalism has led us. And I will raise just one objection to phenomenalism. It is a "reductio ad absurdum," which means an argument where we assume a claim for the sake of argument, arrive at an absurd result, and then conclude the thing we originally assumed had to have been wrong, since it gave us this absurd result.

So here's the objection. Suppose, just for sake of argument, that the phenomenalist is correct. So for me, the physical world is all just a construction out of my sense-data. Now suppose we are walking around University Hall, 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 now notice, we are assuming that phenomenalism is true. But that means that when I see your body, I'm not seeing an irreducibly physical body; I am seeing a bundle of sense-data in my own mind. And let's even 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; but all those intelligent things you say are, after all, sense-data in my own mind. 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!

And 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 outside his own mind. But that means that all other bodies and other minds are part of the external world. And so the phenomenalist is forced to an absurd conclusion. The argument in brief is:

1. If phenomenalism is true, then nothing that is thought to be in the external world exists.

2. But other minds besides my own are thought to be in the external world (since the external world is anything outside my own mind).

3. Therefore, other minds do not exist.

The phenomenalist ends up having to believe solipsism, which is the (absurd) 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. But solipsism is absurd. If phenomenalism would force us to accept solipsism, then we had better reject phenomenalism.

This argument may not be completely fair to the phenomenalist; but I'm not going to spend the time examining those issues any further. I'm just going to assume that phenomenalism has gone seriously wrong.

And indeed, I am going to give another reductio ad absurdum argument, one that is given by a different Scottish philosopher, who lived at the same time as Hume, namely Thomas Reid. 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 we must reject the theory that would force to either skepticism or phenomenalism. That is, we must reject representationalism[?].

Well, what would it mean to reject representationalism? It would mean 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, and which can be defined as follows:

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

Now, don't confuse direct realism with the more na\x{00EF}ve view I talked about earlier, that the world is exactly the way we perceive it to be. Obviously, sometimes we misperceive the world. The direct realist doesn't deny that there are perceptual illusions. The claim is, rather, simply that when we do perceive something, what we directly perceive, the immediate object 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, after all, the whole point of the argument from illusion was to show the need to posit sense-data as the immediate objects of perception. So the last thing we have to do is to show how direct realism might reply to the argument from illusion. So that's what we'll do.

And basically the strategy we'll take is to show how all those different cases of misperception, failed perception, and perceptual relativity -- all those hard cases we gave -- do not really make it necessary to suppose that there are sense-data. We can do a good job of explaining those cases 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 appearance. But the appearance of a stick isn't a sense-datum in my mind. It's a pattern of light, the sort of things that physicists can study, that arrives at my eye. 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 something. But of what? Why say there are two sense-data? Why not, instead, say that I have two eyes, 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 acts of vision 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 something. So maybe it's elephant sense-data that she is hallucinating. Well, that seems like a pretty tough case to deal with. I mean, it's true: it definitely does seem that there is an object, in some 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 wasn't 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 wouldn't mean that she has such images when she engages in actual sense-perception.

Now, I don't think this is a particularly strong reply. I mean, 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. So I think we should look 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 square popping off and disappearing, and the two sides of the square collapsing together at a point, to make a triangle. Even if it should happen that perception does not involve images, other mental processes, like imagination, certainly seem to.

The topic of mental images[?] 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 some sense, we do indeed 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 objects 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 perceive sensations.