enumerate a few and ask whence they spring, and what purpose they serve?
They spring from a very strong conviction—erected upon the basis of Experience by Faith, but absolutely necessary for healthy life and spontaneous action—that the ordinary inferences which we almost instinctively derive from the report of the senses, are true, that is to say, will correspond to experience; and that we can act upon them without formally reasoning upon them.
Take the following instance. Shut your eyes, and get a friend to prick the back of your hand with the two points of a pair of compasses simultaneously, so that the two points may be about the eighth of an inch apart when they touch you; you will feel—and if you could not correct the inference by the sense of sight, you would infer—that only one point is pricking you. The reason is that the skin of the back of the hand only reports one sensation; and the mind leaps to the conclusion—owing to the multitude of past instances where one sensation has resulted from one object—that, in this instance also, one object alone is producing the sensation. A more curious instance is the following: Place the middle finger over the first finger, and between the two fingers thus interlaced place a single marble or your nose: you will appear to be touching two marbles or two noses. The reason is this: when the two fingers are in their usual position (not thus interlaced) and touching marbles or similar objects, two simultaneous sensations on the right side of the right finger and on the left side of the left finger would always imply two marbles; now you have constrained the two fingers to assume an unusual position where these two simultaneous sensations can be produced by one marble; but you, following custom, would infer the presence of two marbles, if sight, or other evidence, did not shew there was only one.
But illusions from the sense of touch are far less
