Of course, we could select one definite volume in space and call all the consecutive stages which it goes through, its history: it then would be part of its history that a cloud was formed in it, or that a bird passed through it on the wing. But it would hardly be found very suggestive to write the history of space-volumes. In fact, it is to *bodies* in space that all history actually relates, at least indirectly, for even the history of sciences is in some respect the history of men or of books. It may suffice for our analysis to understand here the word body in its popular sense. Now in its relation to bodies history may have the three following aspects, as far as anything more than a simple enumeration comes into account. Firstly, it may relate to one and the same body, the term body again to be understood popularly. So it is when the individual history of the organism is traced from the egg to the adult, or when the history of a cloud or of an island or of a volcano is written. Secondly, the subject-matter of history may be formed by the single units of a consecutive series of bodies following each other periodically. To this variety of history the discoveries of Mendel and his followers would belong in the strictest sense, but so does our hypothetical phylogeny and a great part of the history of mankind. And lastly, there is a rather complicated kind of sequence of which the “history” has actually been written. History can refer to bodies which are in no direct relation with one another, but which are each the effect of another body that belongs to a consecutive series of body-units showing periodicity. This sounds rather complicated; but it is only the strict expression of what is perfectly familiar to you all. Our sentence indeed is simply part of the definition of a history of art or of literature for instance--or, say, of a phylogenetic history of the nests of birds. The single pictures are the subjects of the history of art, and nobody would deny that these pictures are the effects of their painters, and that the painters are individuals of mankind--that is, that they are bodies belonging to a consecutive series of body-units showing periodicity. Of course, it is only improperly that we speak of a history of pictures or of books or of nests. In fact, we are dealing with painters, and with men of letters or of science, and with certain birds, and therefore the third type of history may be reduced to the second. But it was not without value to pursue our logical discrimination as far as possible. So far we have always spoken of history as being more than a mere enumeration, but we have not ascertained what this “more” signifies. It is not very difficult to do so: in fact, there are three different types of history, each of a different degree of importance with respect to the understanding of reality.