=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. [Illustration: FIG. 64.] =Illusions of the First Type.=--One of the oldest instances dates from Aristotle. Cross two fingers and roll a pea, penholder, or other small object between them. It will seem double. Professor Croom Robertson has given the clearest analysis of this illusion. He observes that if the object be brought into contact first with the forefinger and next with the second finger, the two contacts seem to come in at different points of space. The forefinger-touch seems higher, though the finger is really lower; the second-finger-touch seems lower, though the finger is really higher. "We perceive the contacts as double because we refer them to two distinct parts of space." The touched sides of the two fingers are normally not together in space, and customarily never do touch one thing; the one thing which now touches them, therefore, seems in two places, i.e. seems two things.