The familiar psychologocal experiment known to every school-boy, and noted already by Aristotle in the Metaphysica (p. 1011, a 33), has often in late years been made the subject of explanation in physiological books, though with little success, as far as I have seen; the explanation consisting generally in a laboured re-statement of the difficulty. What seems to me the true explanation suggested itself once when I tried the experiment, determined carefully to mark the precise phenomenon. Crossing the second finger backwards over the forefinger of the left hand held vertically with the thumb uppermost, so that the under-side of the second finger (usually in contact with the third finger) rested on the upper-side of the forefinger (side next to thumb) I placed a penholder between them, bringing it first into contact with the second finger only. Causing it then to touch the forefinger also, I was struck by perceiving this second contact coming in, as it were, higher up in space, though the forefinger was then lower down. So when the forefinger was first touched, the contact with the second finger was felt as coming in lower down, though the second finger stood them higher up. The spatial reference is still more distinct when the eyes are shut and the judgment is guided by the character of the touches alone; but the most decisive form of the experiment is with other people's fingers, their eyes being shut and the question being simply put: Does the second contact seem to you to come in higher up or lower down in space than the first? The report is always he same; and the interpretation is obvious. We perceive the contacts as double because we refer them to two distinct parts of space. The upper-side of the forefinger and the under-side of the second finger (sides understood as above) are to us distinct parts of space, because normally these two surfaces are not in contact with one another; and they cannot normally be touched simultaneously except by objects which are, or are held to be, two (supposing, that is, bare contact only). Contrariwise, the under-side of the forefinger and the upper-side of the second, being normally in contact with one another, mean to us one and the same space, so that when they are held apart by aught intervening, the suggestion is of a thing filling one and the same space, in other words, a single thing. It is here implied that every part of the tactile surface has a definite spatial character of its own, and about this as a fact there can be no question, whatever difference of opinion there be as to whether such a character is original or derivative.(11 § 1 ¶ 1)

George Croom Robertson