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 Now, all we know of the consciousness authorises us to entrust this rôle to it, for it is within the range of its habits. We know that change is the law of consciousness, that it is effaced when the excitements are uniform, and is renewed by their differences or their novelty. A continued or too often repeated excitement ceases in time to be perceived. It is to condense these facts into a formula that Bain speaks of the law of relativity of cognition, and, in spite of a few ambiguities on the part of Spencer and of Bain himself in the definition of this law, the formula with the sense I have just indicated is worth preserving.

Let us see what becomes of it, when my hypothesis is adopted. It explains how certain excitements proceeding from the objects—that is to say, forming part of the variable element—cease to be perceived when they are repeated and tend to become constant. A fortiori, it seems to me, should the same law explain how the constant element par excellence, the one which never varies from the first hour, is never perceived. There is, in the concert of the sounds of nature, an accompaniment so monotonous that it is no longer