I can even quote on this point a significant fact: there are observers who are organised in such a way that they especially observe by memory. Placed before the sensorial phenomenon which strikes their senses, they are sometimes amazed, as if hypnotised; they require to get away from it to regain consciousness of themselves, to analyse the fact, and to master it, and it is by means of the memory that they study it, on condition, of course, of afterwards coming back to verify their conclusions by a fresh observation from nature. Will it be said that the physicist, the chemist, or the biologist who follows this slow method, and who thus observes retroactively, practises physics and biology by introspection? Evidently this would be ridiculous.
Conversely, introspection may, in certain cases, adopt the procedure of externospection. No doubt it would be inexact to say that the perception of one of our ideas always takes place through the same mechanism as the perception of one of our sensations. To give an account of what we think does not imply the same work as in the case of what we see; for, generally, our thoughts and our images do not appear to us spontaneously. They are first sought for by us, and are only realised after having been wished for. We go from the vague to the precise, from the confused to the clear; the direction of thought precedes, then, its realisation in images; and the latter, being expected, is necessarily comprehended when it is formed. But we may come across curious circumstances in which it is the image which has precedence over its appearance, and in that case it is exact to say that this uninvoked image must be interpreted and recognised as if it were an external object. In cases of this kind, there passes through our mind something which surprises us. I see, by internal vision, a face with a red nose, and I have to search my memory for a long time, even for days, in order to give precision to the vague feeling that I have seen it before, so as to finally say with confidence, "It is So and So!" Or else I hear in my inner ear a certain voice, with a metallic tone and authoritative inflections: this voice pronounces scientific phrases, gives a series of lectures, but I know not to whom it belongs, and it costs me a long effort to reach the interpretation: it is the voice of M. Dastre! There is, then, a certain space of time, more or less long, in which we can correctly assert that we are not aware of what we are thinking; we are in the presence of a thought in the same state of uncertainty as in that of an external, unknown, and novel object. The labour of classification and of interpretation cast upon us is of the same order; and, when this labour is effected incorrectly, it may end in an illusion. Therefore illusions of thought are quite as possible as illusions of the senses, though rarer for the reasons above stated. But the question of frequency has no theoretical importance.