The argument from illusion is an argument for the existence of sense-
data. It is posed as a criticism of direct realism. Naturally occurring
illusions best illustrate the argument's points, a notable example
concerning a stick: I have a stick, which appears to me to be straight, but
when I hold it underwater it seems to bend and distort. I know that the
stick is straight and that its apparent flexibility is as a result of seeing it
through the water, yet I cannot change the mental image I have of the
stick as being bent. Since the stick is not in fact bent its appearance can
be described as an illusion. Rather than directly perceiving the stick,
which would entail our seeing it as it truly is, we must instead perceive it
indirectly, via a sense-datum. This mental representation does not tell us
anything about the stick's true properties, which remain inaccessible to us.
With this being the case, however, how can we be said to be certain of the
stick's initial straightness? If all we perceive is sense-data then the stick's
apparent initial straightness is just as likely to be false as its half-
submerged bent appearance. Therefore, the argument runs, we can
never gain any knowledge about the stick, as we only ever perceive a
sense-datum, and not the stick itself.
Zhuangzi wrote:He who dreams of drinking wine may weep when morning comes; he who
dreams of weeping may in the morning go off to hunt. While he is
dreaming he does not know it is a dream, and in his dream he may even
try to interpret a dream. Only after he wakes does he know it was a
dream. And someday there will be a great awakening when we know that
this is all a great dream. Yet the stupid believe they are awake, busily and
brightly assuming they understand things, calling this man ruler, that one
herdsman ‑ how dense! Confucius and you are both dreaming! And when
I say you are dreaming, I am dreaming, too. Words like these will be
labeled the Supreme Swindle. Yet, after ten thousand generations, a great
sage may appear who will know their meaning, and it will still be as
though he appeared with astonishing speed.