But since history is the foundation of the natural sciences, and the special treatment of perceptive material or historical data by these sciences does not possess theoretic value, but is valuable merely as a convenient classification, it is clear that the whole content of truth of the natural sciences (the measure of truth and reality that at bottom they contribute) is history. Therefore it is not without reason that the natural sciences or some of them have been called in the past _natural history._ History is the hot and fluid mass, which the naturalist cools and solidifies by pouring it into formal classes and types. Previous to this manipulation, the naturalist must have thought as a historian. The matter thus cooled and solidified for preservation and for transport has no theoretic value, save in so far as it can again be rendered hot and fluid. Similarly, on the other hand, it is necessary to revise continually the classifications adopted, returning to the observation of facts, to simple intuitions and perceptions, to the historical consideration of reality. The _naturalist_ who makes a discovery, in so far as he is a discoverer of truth, is a _historical_ discoverer; and revolutions in the natural sciences represent progress in historical knowledge. Lamarckianism and Darwinism may serve as an example of this. Naturalists (and we use the word in its ordinary meaning, applying it to those who explore this "fair family of plants and animals," and what is called in general the physical world) feel themselves somewhat humiliated when described as classifiers careless of truth. But if such classification is exactly what the natural sciences accomplish from the gnoseological point of view, yet naturalists as individuals and as corporations of students exercise a far more substantial and fruitful function. The historical foundation of the life of the natural sciences is also found in the fact that a change of historical conditions sometimes renders, if not wholly useless, at least less useful, certain classifications made with the object of controlling conditions of life remote from us, or perceptions concerning life that have now been abandoned. This has occurred with regard to the classifications of alchemy and of astrology, and also (passing on to examples from other empirical sciences) to the descriptive and casuistic portions of feudal law. When the book is no longer read, the _index_ also falls into disuse. [Sidenote: _The prejudice as to the non-historicity of nature._]