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Philosophical Prestidigitation

by Joel Marks

Back in Issue 48 I wrote about my fascination with vision. There is much more to say. I left off with the tantalizing suggestion that everything that we see is in some sense ‘in our mind.’ This was a version of the Argument from Illusion. A simple example is this. You are experiencing a visual illusion, such as the shaken pencil that appears to be rubbery. Suppose you see me performing this trick. You see the ‘rubbery pencil,’ which you know is in fact a rigid pencil (although the illusion can be convincing enough to make you doubt your knowledge, which after all could be false if I had been trying to fool you all along). But let’s say it really is rigid.

But what is the ‘it’ that is rigid? The pencil itself, of course. But what are you seeing? What you are seeing is decidedly flexible, not rigid. So it seems natural to conclude that what you are seeing is not the pencil at all, but only a visual image of the pencil. Such an image used to be called a ‘sense datum’ in philosophical circles.

Now, the ‘So’ in the preceding paragraph may also be a little sleight of hand, since its logic is suspect. Since the word is so small, however, it is easy to slip it into the magician’s patter and make you think you have concluded something significant. Anyhow, the Argument from Illusion proceeds. Since the pencil you are seeing is not a pencil at all but an image of a pencil, then we suddenly face the peculiar problem: What is holding the pencil?

Why, my hand, of course. My hand looks its normal self – no fluid hand where there should be a solid one, for instance. Yet, the hand is holding a pencil that isn’t there ... and there does not appear to be any gap – visual or real – between the pencil in the hand and the hand itself. So ... might not one conclude that the hand you see is also not a hand, but just the image of a hand?

If so, then it’s a short hop of inference to conclude that everything you, or anyone, ever sees is the content of one’s own mental experience and not a part of the physical world at all. Indeed, I can make you disappear – that is, cease to exist in the material world – simply by touching you with my hand! For if my hand – that is, the visual image of the hand that you see – is not ‘really there’ in the physical world, then – just as the imaginal pencil ‘infected’ the hand with its immateriality – so the hand in turn can infect you (that is, your body) with its!

(This traveling infection reminds me of the dreaded Ice Nine in Kurt Vonnegut’s novel, Cat’s Cradle. I will not spoil the ending of the book for those of you who have not read it by revealing the details of this analogy.)

But now let me reverse the trick with a continuation of this visual dialectic. This time I will make the image itself vanish from existence and leave in its place the good old physical world. The previous demonstration would seem to have established that certainty resides in our own (in this case, visual) experience. It might even be possible to doubt or deny the very existence of a material world beyond our own mental experience (as the classical idealists, such as Bishop Berkeley, did).

But consider this argument. Imagine a nice red apple. Now there seems to be no question at all of this being a physical apple, because this is something you have just summoned up in your mind. You could even have your eyes closed. If you know anything for certain, it is that there is an image of an apple in the universe ... even if everything else, including your own body, should prove to be hallucinatory.

Now let me ask you a simple question: Is that ‘apple’ red? At first it seemed simple enough to assert that it is not an apple; there is no piece of fruit inside your skull. But why, then, are you so sure that there is anything red there?

Well, you say, the redness is not inside my skull – no more than is the ‘apple’. Both are in my mind, which, being by definition non-physical, is not located anywhere in physical space, including inside my skull.

Curious that the apple image should reside in time, though, is it not? That is, you are experiencing it right now and not an hour earlier or later; but is not time also a physical phenomenon?

More directly to the point: What is red; that is, redness? If we take the redness of an apple as paradigmatic, then is it not natural to infer that the color is a quality of the apple – that is, of a real, physical apple? In other words, the very notion of red is something we know about from acquaintance with the surface of a physical object; it is, perhaps, a particular chemical composition of a particular substance that alters incoming ‘white’ light in such a way as to emit radiation that then impinges on the light-sensitive cells of our retinae and ultimately activates the optic cortex in such a way that we have a certain experience that we have learned to label ‘red’. Yes?

If that is so, then it is clear that there is no red image in your mind at all right now, because there is (presumably) no apple skin, or anything comparable, inside your skull responsible for the experience you are having when you summon up a red apple in imagination. Presto! No red apple (image)! And by analogous argument, no mental apple at all.

What there is, then, is some sort of brain event, no doubt comparable to the event in your optic cortex when you are seeing a real red apple (or bloody dagger) before you.

So much for philosophical prestidigitation. Let me end by noting that vision, even if purely physical, still contains magic – real magic, if I may use that term. For example, when you look at the Andromeda Nebula on a clear, moonless night, with only your naked eyes you can literally see 14 trillion miles into the distance and two million years into the past.

© Joel Marks 2005

Joel Marks is Professor of Philosophy at the University of New Haven in West Haven, Connecticut.


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