Sensual representations are in a perpetual flux; they come after each other like the waves of a river, and even during the time that they last they do not remain the same thing. Each of them is an integral part of the precise instant when it takes place. We are never sure of again finding a perception such as we experienced it the first time; for if the thing perceived has not changed, it is we who are no longer the same. On the contrary, the concept is, as it were, outside of time and change; it is in the depths below all this agitation; it might be said that it is in a different portion of the mind, which is serener and calmer. It does not move of itself, by an internal and spontaneous evolution, but, on the contrary, it resists change. It is a manner of thinking that, at every moment of time, is fixed and crystallized. In so far as it is what it ought to be, it is immutable. If it changes, it is not because it is its nature to do so, but because we have discovered some imperfection in it; it is because it had to be rectified. The system of concepts with which we think in everyday life is that expressed by the vocabulary of our mother-tongue; for every word translates a concept. Now language is something fixed; it changes but very slowly, and consequently it is the same with the conceptual system which it expresses. The scholar finds himself in the same situation in regard to the special terminology employed by the science to which he has consecrated himself, and hence in regard to the special scheme of concepts to which this terminology corresponds. It is true that he can make innovations, but these are always a sort of violence done to the established ways of thinking. And at the same time that it is relatively immutable, the concept is universal, or at least capable of becoming so. A concept is not my concept; I hold it in common with other men, or, in any case, can communicate it to them. It is impossible for me to make a sensation pass from my consciousness into that of another; it holds closely to my organism and personality and cannot be detached from them. All that I can do is to invite others to place themselves before the same object as myself and to leave themselves to its action. On the other hand, conversation and all intellectual communication between men is an exchange of concepts. The concept is an essentially impersonal representation; it is through it that human intelligences communicate.