Illusions are not Perceptual Errors

The senses don't distort, this fact is not negated by the existence of illusions such as mirages or the classic one of a stick appearing to bend as it enters water. In such situations the senses have not invented or distorted anything; they are correctly relaying the data coming in---man's eyes in the case of the stick going into water or the mirage in the desert did not invent the light rays coming in. If a man incorrectly concludes that there is an oasis where he sees a mirage the fault is not present with his eyes lying to him, rather the fault is in his identification of the mirage as an oasis---namely it is at the conceptual level that the error is present. The deal is the same as with the stick "bending" in water---the light rays are genuinely bending as they enter the water due to the phenomenon of refraction, and man's eyes relay this information to him. It is the fault of his reason if he concludes that it is the stick which is bending and not the light rays.

A so-called sensory illusion, such as a stick in water appearing bent, is not a perceptual error. In Ayn Rand's view, it is a testament to the reliability of the senses. The senses do not censor their response; they do not react to a single attribute (such as shape) in a vacuum, as though it were unconnected to anything else; they cannot decide to ignore part of the stimulus. Within the range of their capacity, the senses give us evidence of everything physically operative, they respond to the full context of the facts---including, in the present instance, the fact that light travels through water at a different rate than through air, which is what causes the stick to appear bent. It is the task not of the senses but of the mind to analyze the evidence and identify the causes at work (which may require the discovery of complex scientific knowledge). If a casual observer were to conclude that the stick actually bends in water, such a snap judgment would be a failure on the conceptual level, a failure of thought, not of perception. To criticize the senses for it is tantamount to criticizing them for their power, for their ability to give us evidence not of isolated fragments, but of a total.^[1]