Fig. 63.
Illusions.—Let us now, for brevity's sake, treat A and B in Fig. 63 as if they stood for objects instead of brain-processes. And let us furthermore suppose that A and B are, both of them, objects which might probably excite the sensation which I have called 'this,' but that on the present occasion A and not B is the one which actually does so. If, then, on this occasion 'this' suggests A and not B, the result is a correct perception. But if, on the contrary, 'this' suggests B and not A, the result is a false perception, or, as it is technically called, an illusion. But the process is the same, whether the perception be true or false.
Note that in every illusion what is false is what is inferred, not what is immediately given. The 'this,' if it were felt by itself alone, would be all right; it only becomes misleading by what it suggests. If it is a sensation of sight, it may suggest a tactile object, for example, which later tactile experiences prove to be not there. The so-called 'fallacy of the senses,' of which the ancient sceptics made so much account, is not fallacy of the senses proper, but rather of the intellect, which interprets wrongly what the senses give.[39]
So much premised, let us look a little closer at these illusions. They are due to two main causes. The wrong object is perceived either because
1) Although not on this occasion the real cause, it is yet the habitual, inveterate, or most probable cause of 'this'; or because
2) The mind is temporarily full of the thought of that object, and therefore 'this' is peculiarly prone to suggest it at this moment.
I will give briefly a number of examples under each head. The first head is the more important, because it includes a number of constant illusions to which all men are subject, and which can only be dispelled by much experience.
Fig. 64.