Task:

In your paper I would like you to discuss and evaluate the following thesis about visual perception:

• (SD) Whenever one has a visual experience (of, say, a cat), what one sees directly or immediately is a cat-like sense datum, and not a cat. 

Specifically, drawing on Chapter III of JL Austin’s Sense and Sensibilia (file: JL Austin Sense and Sensibilia, Ch III ), your essay should do the following three things:

• Carefully, and in your own words, present one version of the “Argument from Illusion” for (SD) that Austin discusses. 
• Carefully, and in your own words, present Austin’s response to that argument. 
• Discuss which argument is superior: the Argument from Illusion, or Austin’s response. 

Specs:

• Your paper should be 800-1000 words.
• You do not need to include a bibliography; simply reference page numbers where appropriate. 
• Your paper will be marked according to this rubric. 
• See Jim Pryor’s Guidelines on Writing a Philosophy Paper (Links to an external site.) if you aren’t sure how to begin. (It is a good idea for everyone to read these Guidelines!) (Guideline: http://www.jimpryor.net/teaching/guidelines/writing.html)

Guidelines for Writing Exercises

Nate Charlow

January

1

9, 2021

Grading. 100 points are possible for each writing exercise. Here is how those points are assigned.

• Mastery of the text/issue (40). Excellent writing demonstrates a solid and sophisticated
understanding of the relevant philosophical text/issue, the evidence of careful reading and
study.

• Clarity, rigor, and care of presentation (40). Excellent writing clearly articulates
central philosophical claims (and clearly explains how the arguments for these claims work);
extraneous detail is omitted; a simple and compelling presentation of a philosophical point-
of-view is achieved.

• Writing and organization (20 points). Excellent writing uses transparent and easy-to-
read prose; it is concise, precise, and pleasant/straightfoward to read.

Grading Table

Mastery of the text (40 possible) 40 36 32 28 24 20 16

Clarity, rigor, compellingness (40 possible) 40 36 32 28 24 20 16

Writing and organization (20 possible) 20 18 16 14 12 10 8

More information. Here is a useful general resource on writing philosophy papers, written by
Jim Pryor. Note that some of Pryor’s advice is only appropriate for longer-form philosophical
writing. But a good deal of this advice—for example: use simple prose, make the structure of
your writing obvious, explains key claims and terms concisely but also fully—is useful for even
short-form philosophical writing (like the kind you’ll be doing in the Writing Exercises).

1

http://www.jimpryor.net/teaching/guidelines/writing.html

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.