_Thus the faintest sensations will give rise to the perception of definite things if only they resemble those which the things are wont to arouse._ In fact, a sensation must be strong and distinct in order not to suggest an object and, if it is a nondescript feeling, really to seem one. The auræ of epilepsy, globes of light, fiery vision, roarings in the ears, the sensations which electric currents give rise to when passed through the head, these are unfigured because they are strong. Weaker feelings of the same sort would probably suggest objects. Many years ago, after reading Maury's book, _Le Sommeil et les Rêves_, I began for the first time to observe those ideas which faintly flit through the mind at all times, words, visions, etc., disconnected with the main stream of thought, but discernible to an attention on the watch for them. A horse's head, a coil of rope, an anchor, are, for example, ideas which have come to me unsolicited whilst I have been writing these latter lines. They can often be explained by subtle links of association, often not at all. But I have not a few times been surprised, after noting some such idea, to find, on shutting my eyes, an after-image left on the retina by some bright or dark object recently looked at, and which had evidently suggested the idea. 'Evidently,' I say, because the general shape, size, and position of object thought-of and of after-image were the same, although the idea had details which the retinal image lacked. We shall probably never know just what part retinal after-images play in determining the train of our thoughts. Judging by my own experiences I should suspect it of being not insignificant.[89] ILLUSIONS. Let us now, for brevity's sake, treat A and B in Fig. 47 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._[90] * * * * *