The empirical character (and the practical character in the sense already established) of the natural sciences is commonly admitted in the case of such of them as consist in classifications of facts: for example, of zoology, botany, mineralogy, and also of chemistry, in so far as it enumerates chemical species, and of physics, in so far as it enumerates classes of phenomena or physical forces. The universals of all these sciences are quite arbitrary, for it is impossible to find an exact boundary between the concept of animal (the universal of zoology) and that of vegetable (the universal of botany). Indeed it is impossible to find one between the living and the not living, the organic and the material. Finally, the cellule, which is, for the present at any rate, the highest concept of the biological sciences, is differentiated from chemical facts only in an external way. It will be objected that there is in any case no lack of attempts to determine strictly the supreme concepts of the sciences, such, for instance, as those that place the _atom_ at the beginning of all things and attempt to show each individual fact as nothing but a different aggregate of atoms. There are also those who mount to the concept of _ether_ or of _energy_ and declare all individual facts to be nothing but different forms of energy. Or finally, the vitalists recognize as irreducible the two concepts of the teleological and the mechanical, of organic and inorganic, of life and matter. But in all these cases _the natural sciences are deserted,_ phenomena are abandoned for noumena, and philosophic explanations are offered. These may or may not have value, but they are of no use from the point of view of the natural sciences, or at most ensure to some professor the insipid pleasure of calling an animal "a complex of atoms," heat "a form of energy," and the cellule "vital force." [Sidenote: _Impossibility of introducing into them strict divisions._]