Coffey: Senses and Intellect #2

 As noted in my prior post, Peter Coffey mentioned the straight stick partly submerged in water on page 237 of Volume 1 of Epistemology; Or the Theory of Knowledge. He addresses said stick again in Volume 2 as follows.

"But the presentation of a sense datum can be an occasion of deception to the perceiver, inasmuch as the latter may judge that the datum is external, or how it is externally, without adverting to the fact that the presented datum is partially determined by the conditions of his organism, and this condition is, perhaps, abnormal. And just as the subjective, organic condition of the perceiver may be an occasion of error in his spontaneous judgment, so may the abnormal condition of the external thing itself, or the physical medium spatially intervening between the latter and the sense organ of the perceiver. A trite and telling example of this source of error is the familiar fact that a straight stick partially immerses in water and seen obliquely appears bent. Or again, to a person sitting in a moving train which is passing another train that is stationary, the latter appears to be moving and the former at rest: a double or compound illusion. Or, an object seen through a microscope appears much larger than "it really is". [ ]
   "Now such "illusions of the senses," though puzzling to the plain man, have never shaken his spontaneous belief in the trustworthiness of his senses under normal conditions. But philosophers, who have tried to think out the bearing of these illusions on our spontaneous beliefs regarding the existence, qualities, and nature of the external domain of reality, have been more than puzzled by such illusions: many have been driven by them into the position of theoretical scepticism, subjectivism or idealism" (p. 93-4).

Straight trumps bent because the stick is seen as straight when not in the water or fully immersed in the water, and can be felt as straight even while seen as bent. We judge that under most and normal conditions perception matches reality. However, such analysis does not apply to a circular table top. Under most and normal conditions the table top is seen as elliptical rather than circular, and we judge it to be really circular. On the other hand, we don't regard the elliptical case as an illusion. In every case what is seen conforms to and depends on the retinal image.

These example, the trains, the moon seeming almost as large as the sun, the railroad tracks that seem to converge, and others seem to support no comprehensive rule for identifying illusory versus non-illusory percepts.