Philosophy paper

III

T HE PRIMARY PURPOSE OF THE ARGUMENT from illusion is to induce people to accept 'sense-data' as the proper and correct answer to the question what they perceive on certain abnormal, exceptional occasions; but in fact it is usually followed up with another bit of argument intended to establish that they always perceive sense-data. Well, what is the argument?

In Ayer's statement1 it runs as follows. It is 'based on the fact that material things may present different ap- pearances to different observers, or to the same observer in different conditions, and that the character of these appearances is to some extent causally determined by the state of the conditions and the observer'. As illustrations of this alleged fact Ayer proceeds to cite perspective ('a coin which looks circular from one point of view may look elliptical from another'); refraction ('a stick which normally appears straight looks bent when it is seen in water'); changes in colour-vision produced by drugs ('such as mescal'); mirror-images; double vision; halluci- nation; apparent variations in tastes; variations in felt warrnth ('according as the hand that is feeling it is itself

1 Ayer, op. cit., pp. 3-5.

Sense and Sensibilia 21 hot or cold'); variations in felt bulk ('a coin seems larger when it is placed on the tongue than when it is held in the palm of the hand'); and the oft-cited fact that 'people who have had limbs amputated may still continue to feel pain in them'.

He then selects three of these instances for detailed treatment. First, refraction-the stick which normally 'appears straight' but 'looks bent' when seen in water. He makes the 'assumptions' (a) that the stick does not really change its shape when it is placed in water, and (b) that it cannot be both crooked and straight. 1 He then concludes ('it follows') that 'at least one of the visual appearances of the stick is delusive'. Nevenheless, even when 'what we see is not the real quality of a material thing, it is supposed that we are still seeing something' -and this something is to be called a 'sense-datum'. A sense-datum is to be 'the object of which we are directly aware, in perception, if it is not part of any material thing'. (The italics are mine throughout this and the next two paragraphs.)

Next, mirages. A man who sees a mirage, he says, is 'not perceiving any material thing; for the oasis which he thinks he is perceiving does not exist'. But 'his ex- perience is not an experience of nothing'; thus 'it is said that he is experiencing sense-data, which are similar in character to what he would be experiencing if he were seeing a real oasis, but are delusive in the sense that the

1 It is not only strange, but also important, that Ayer calls these 'as- sumptions'. Later on he is going to take seriously the notion of denying at least one of them, which he could hardly do if he had recognized them here as the plain and incontestable facts that they are.

22 Sense and Sensibilia material thing which they appear to present is not really there'.

Lastly, reflections. When I look at myself in a mirror 'my body appears to be some distance behind the glass'; but it cannot actually be in two places at once; thus, my perceptions in this case 'cannot all be veridical'. But I do see something; and if 'there really is no such material thing as my body in the place where it appears to be, what is it that I am seeing?' Answer-a sense-datum. Ayer adds that 'the same conclusion may be reached by taking any other of my examples'.

Now I want to call attention, first of all, to the name of this argument-the 'argument from illusion', and to the fact that it is produced as establishing the conclusion that some at least of our 'perceptions' are delusive. For in this there are two clear implications-(a) that all the cases cited in the argument are cases of illusions; and (b) that illusion and delusion are the same thing. But both of these implications, of course, are quite wrong; and it is by no means unimportant to point this out, for, as we shall see, the argument trades on confusion at just this point.

What, then, would be some genuine examples of illu- sion? (The fact is that hardly any of the cases cited by Ayer is, at any rate without stretching things, a case of illusion at all.) Well, first, there are some quite clear cases of optical illusion-for instance the case we mentioned earlier in which, of two lines of equal length, one is made to look longer than the other. Then again there are illusions produced by professional 'illusionists', conjurors

r Sense and Sensibilia 23

-for instance the Headless Woman on the stage, who is made to look headless, or the ventriloquist's dummy which is made to appear to be talking. Rather different-not (usually) produced on purpose-is the case where wheels rotating rapidly enough in one direction may look as if they were rotating quite slowly in the opposite direction. Delu- sions, on the other hand, are something altogether dif- ferent from this. Typical cases would be delusions of persecution, delusions of grandeur. These are primarily a matter of grossly disordered beliefs (and so, probably, behaviour) and may well have nothing in particular to do with perception. 1 But I think we might also say that the patient who sees pink rats has (suffers from) delusions- particularly, no doubt, if, as would probably be the case, he is not clearly aware that his pink rats aren't real rats.:z.

The most important differences here are that the term 'an illusion' (in a perceptual context) does not suggest that something totally unreal is conjured up-on the con- trary, there just is the arrangement of lines and arrows on the page, the woman on the stage with her head in a black bag, the rotating wheels; whereas the term 'de- lusion' does suggest something totally unreal, not really there at all. (The convictions of the man who has delu- sions of persecution can be completely without founda- tion.) For this reason delusions are a much more serious matter-something is really wrong, and what's more,

1 The latter point holds, of course, for some uses of 'illusion' too; there are the illusions which some people (are said to) lose as they grow older and wiser.

2 Cp. the white rabbit in the play called Harvey.

24 Sense and Sensibilia wrong with the person who has them. But when I see an optical illusion, however well it comes off, there is nothing wrong with me personally, the illusion is not a little (or a large) peculiarity or idiosyncrasy of my own; it is quite public, anyone can see it, and in many cases standard procedures can be laid down for producing it. Furthermore, if we are not actually to be taken in, we need to be on our guard; but it is no use to tell the suf- ferer from delusions to be on his guard. He needs to be cured.

Why is it that we tend-if we do-to confuse illu- sions with delusions? Well, partly, no doubt the terms are often used loosely. But there is also the point that people may have, without making this explicit, different views or theories about the facts of some cases. Take the case of seeing a ghost, for example. It is not generally known, or agreed, what seeing ghosts is. Some people think of seeing ghosts as a case of something being con- jured up, perhaps by the disordered nervous system of the victim; so in their view seeing ghosts is a case of de- lusion. But other people have the idea that what is called seeing ghosts is a case of being taken in by shadows, per- haps, or reflections, or a trick of the light-that is, they assimilate the case in their minds to illusion. In this way, seeing ghosts, for example, may come to be labelled sometimes as 'delusion', sometimes as 'illusion'; and it may not be noticed that it makes a difference which label we use. Rather, similarly, there seem to be different doc- trines in the field as to what mirages are. Some seem to

Sense and Sensibilia 25 take a mirage to be a vision conjured up by the crazed brain of the thirsty and exhausted traveller (delusion), while in other accounts it is a case of atmospheric refrac- tion, whereby something below the horizon is made to appear above it (illusion). (Ayer, you may remember, takes the delusion view, although he cites it along with the rest as a case of illusion. He says not that the oasis appears to be where it is not, but roundly that 'it does not exist'.)

The way in which the 'argument from illusion' posi- tively trades on not distinguishing illusions from delu- sions is, I think, this. So long as it is being suggested that the cases paraded for our attention are cases of illusion, there is the implication (from the ordinary use of the word) that there really is something there that we per- ceive. But then, when these cases begin to be quietly called delusive, there comes in the very different sugges- tion of something being conjured up, something unreal or at any rate 'immaterial'. These two implications taken together may then subtly insinuate that in the cases cited there really is something that we are perceiving, but that this is an immaterial something; and this in- sinuation, even if not conclusive by itself, is certainly well calculated to edge us a little closer towards just the position where the sense-datum theorist wants to have us.

So much, then-though certainly there could be a good deal more-about the differences between illusions and delusions and the reasons for not obscuring them. Now let us look briefly at some of the other cases Ayer

26 Sense and Sensibilia lists. Reflections, for instance. No doubt you can produce illusions with mirrors, suitably disposed. But is just any case of seeing something in a mirror an illusion, as he implies? Quite obviously not. For seeing things in mirrors is a perfectly normal occurrence, completely familiar, and there is usually no question of anyone being taken in. No doubt, if you're an infant or an aborigine and have never come across a mirror before, you may be pretty baffled, and even visibly perturbed, when you do. But is that a reason why the rest of us should speak of illusion here? And just the same goes for the phenomena of perspec- tive-again, one can play tricks with perspective, but in the ordinary case there is no question of illusion. That a round coin should 'look elliptical' (in one sense) from some points of view is exactly what we expect and what we normally find; indeed, we should be badly put out if we ever found this not to be so. Refraction again-the stick that looks bent in water-is far too familiar a case to be properly called a case of illusion. We may perhaps be prepared to agree that the stick looks bent; but then we can see that it's partly submerged in water, so that is exactly how we should expect it to look.

It is important to realize here how familiarity, so to speak, takes the edge off illusion. Is the cinema a case of illusion? Well, just possibly the first man who ever saw moving pictures may have felt inclined to say that here was a case of illusion. But in fact it's pretty unlikely that even he, even momentarily, was actually taken in; and by now the whole thing is so ordinary a part of our lives

Sense and Sensibilia 27 that it never occurs to us even to raise the question. One might as well ask whether producing a photograph is producing an illusion-which would plainly be just silly.

Then we must not overlook, in all this talk about illu- sions and delusions, that there are plenty of more or less unusual cases, not yet mentioned, which certainly aren't either. Suppose that a proof-reader makes a mistake-he fails to notice that what ought to be 'causal' is printed as 'casual'; does he have a delusion? Or is there an illusion before him? Neither, of course; he simply misreads. Seeing after-images, too, though not a particularly fre- quent occurrence and not just an ordinary case of seeing, is neither seeing illusions nor having delusions. And what about dreams? Does the dreamer see illusions? Does he have delusions? Neither; dreams are dreams.

Let us tum for a moment to what Price has to say about illusions. He produces, 1 by way of saying 'what the term "illusion" means', the following 'provisional definition': 'An illusory sense-datum of sight or touch is a sense- datum which is such that we tend to take it to be part of the surface of a material object, but if we take it so we are wrong.' It is by no means clear, of course, what this dictum itself means; but still, it seems fairly clear that the definition doesn't actually fit all the cases of illusion. Consider the two lines again. Is there anything here which we tend to take, wrongly, to be part of the surface of a material object? It doesn't seem so. We just see the two lines, we don't think or even tend to think that we

1 Perception, p. 27.

28 Sense and Sensibilia see anything else, we aren't even raising the question whether anything is or isn't 'part of the surface' of- what, anyway? the lines ? the page ?-the trouble is just that one line looks longer than the other, though it isn't. Nor surely, in the case of the Headless Woman, is it a question whether anything is or isn't part of her surface; the trouble is just that she looks as if she had no head.

It is noteworthy, of course, that, before he even begins to consider the 'argument from illusion', Price has al- ready incorporated in this 'definition' the idea that in such cases there is something to be seen in addition to the ordinary things-which is part of what the argument is commonly used, and not uncommonly taken, to prove. But this idea surely has no place in an attempt to say what 'illusion' means. It comes in again, improperly I think, in his account of perspective (which incidentally he also cites as a species of illusion)-'a distant hillside which is full of protuberances, and slopes upwards at quite a gentle angle, will appear flat and vertical. ... This means that the sense-datum, the colour-expanse which we sense, actually is flat and venical.' But why should we accept this account of the matter? Why should we say that there is anything we see which is flat and vertical, though not 'part of the surface' of any material object? To speak thus is to assimilate all such cases to cases of delusion, where there is something not 'part of any material thing'. But we have already discussed the un- desirability of this assimilation.

Next, let us have a look at the account Ayer himself

Sense and Sensibilia 29 gives of some at least of the cases he cites. (In fairness we must remember here that Ayer has a number of quite substantial reservations of his own about the merits and efficacy of the argument from illusion, so that it is not easy to tell just how seriously he intends his exposition of it to be taken; but this is a point we shall come back to.)

First, then, the familiar case of the stick in water. Of this case Ayer says (a) that since the stick looks bent but is straight, 'at least one of the visual appearances of the stick is delusive'; and (b) that 'what we see [directly any- way] is not the real quality of [a few lines later, not part of] a material thing'. Well now: does the stick 'look bent' to begin with? I think we can agree that it does, we have no better way of describing it. But of course it does not look exactly like a bent stick, a bent stick out of water-at most, it may be said to look rather like a bent stick partly immersed in water. After all, we can't help seeing the water the stick is partly immersed in. So exactly what in this case is supposed to be delusive? What is wrong, what is even faintly surprising, in the idea of a stick's being straight but looking bent sometimes? Does anyone sup- pose that if something is straight, then it jolly well has to look straight at all times and in all circumstances? Obviously no one seriously supposes this. So what mess are we supposed to get into here, what is the difficulty ? For of course it has to be suggested that there is a diffi- culty-a difficulty, furthermore, which calls for a pretty radical solution, the introduction of sense-data. But what is the problem we are invited to solve in this way?

30 Sense and Sensibilia Well, we are told, in this case you are seeing some-

thing; and what is this something 'if it is not part of any material thing,? But this question is, really, completely mad. The straight part of the stick, the bit not under water, is presumably part of a material thing; don,t we see that? And what about the bit under water ?-we can see that too. We can see, come to that, the water itself. In fact what we see is a stick partly immersed in water; and it is particularly extraordinary that this should appear to be called in question-that a question should be raised about what we are seeing-since this, after all, is simply the description of the situation with which we started. It was, that is to say, agreed at the start that we were looking at a stick, a 'material thing', part of which was under water. If, to take a rather different case, a church were cunningly camouflaged so that it looked like a barn, how could any serious question be raised about what we see when we look at it ? We see, of course, a church that now looks like a barn. We do not see an immaterial barn, an immaterial church, or an immaterial anything else. And what in this case could seriously tempt us to say that we do?

Notice, incidentally, that in Ayer's description of the stick-in-water case, which is supposed to be prior to the drawing of any philosophical conclusions, there has al- ready crept in the unheralded but important expression 'visual appearances'-it is, of course, ultimately to be suggested that all we ever get when we see is a visual appearance (whatever that may be).

Consider next the case of my reflection in a mirror.

Sense and Sensibilia 31 My body, Ayer says, 'appears to be some distance behind the glass'; but as it's in front, it can't really be behind the glass. So what am I seeing? A sense-datum. What about this? Well, once again, although there is no objection to saying that my body 'appears to be some distance behind the glass', in saying this we must remember what sort of situation we are dealing with. It does not 'appear to be' there in a way which might tempt me (though it might tempt a baby or a savage) to go round the back and look for it, and be astonished when this enterprise proved a failure. (To say that A is in B .doesn't always mean that if you open B you will find A, just as to say that A is on B doesn't always mean that you could pick it off-consider 'I saw my face in the mirror', 'There's a pain in my toe', 'I heard him on the radio', 'I saw the image on the screen', &c. Seeing something in a mirror is not like seeing a bun in a shop-window.) But does it follow that, since my body is not actually located behind the mirror, I am not seeing a material thing? Plainly not. For one thing, I can see the mirror (nearly always anyway). I can see my own body 'indirectly', sc. in the mirror. I can also see the re- flection of my own body or, as some would say, a mirror- image. And a mirror-image (if we choose this answer) is not a 'sense-datum'; it can be photographed, seen by any number of people, and so on. (Of course there is no question here of either illusion or delusion.) And if the question is pressed, what actually is some distance, five feet say, behind the mirror, the answer is, not a sense- datum, but some region of the adjoining room.

32 Sense and Sensibilia The mirage case-at least if we take the view, as Ayer

does, that the oasis the traveller thinks he can see 'does not exist' -is significantly more amenable to the treat- ment it is given. For here we are supposing the man to be genuinely deluded, he is not 'seeing a material thirig,. 1 We don,t actually have to say, however, even here that he is 'experiencing sense-data,; for though, as Ayer says above, 'it is convenient to give a name, to what he is ex- periencing, the fact is that it already has a name-a mirage. Again, we should be wise not to accept too readily the statement that what he is experiencing is 'similar in character to what he would be experiencing if he were seeing a real oasis,. For is it at all likely, really, to be very similar? And, looking ahead, if we were to concede this point we should find the concession being used against us at a later stage-namely, at the stage where we shall be invited to agree that we see sense-data always, in normal cases too.

1 Not even 'indirectly', no such thing is 'presented'. Doesn't this seem to make the case, though more amenable, a good deal less useful to the philosopher? It's hard to see how normal cases could be said to be very like this.