It is not surprising that biology, even more than physics and chemistry, preserves the metaphysical spirit. Such, for instance, is the hypothesis of spontaneous generation. Positive philosophy recognizes that each living being always emanates from another similar being. This is not established _a priori_, but is the result of an “immense induction.”[175] _Omne vivum ex vivo._ Efforts to explain how the generating tissue should itself be formed by kinds of organic monads, (an allusion to certain theories arising out of Schelling’s philosophy) can only fail. We should never know how to connect the organic with the inorganic world except through the fundamental laws belonging to the general phenomena which are common to them both. Positive speculations in anatomy and in physiology form a limited system, within which we must establish the most perfect unity, but which must ever remain separated from the whole of inorganic theories.[176] We see clearly, it is true, that there is no matter which is of itself living. Life is not peculiar to certain substances which are organised in a certain manner. It never belongs to them for more than a time: every organism of which the molecules are not renewed is dissolved. But “we can no more explain this instability than this speciality.”[177] In the same way we see that in living bodies the nutritive functions are the basis of the others; but there is no contradiction in “dreaming” of thought and sociability in beings whose substance would remain unalterable. From this point of view spiritualism is not less admissible than materialism, in so much as death does not seem to be a necessary consequence of life. This again is an idea which is common to Descartes and to Comte. They both conceive an organism in which the play of functions should not cease of itself. The theory of death, says Comte, although it is founded upon that of life, is entirely distinct from it.[178] If biology still often hesitates in the statement of its problems and in the choice of its hypotheses, it is in a great measure due to the two opposite tendencies between which it oscillated in the last century. On the one hand, Boerhaave, and the school of physiology which is more or less directly connected with Descartes, sought a mechanical explanation of biological phenomena, and tended to reduce biology to physics and chemistry. On the other hand, Stahl in Germany, and the vitalist school of Montpellier in France, appealed to metaphysical principles and to unverifiable hypotheses. Being thus swayed from one extremity to another, biology only escaped the “oppression” of the inorganic sciences to involve itself in conceptions which were scarcely scientific.[179] It is only at the end of the XVIII. Century, with Haller, Gall and Bichat, that it finds its equilibrium, takes possession of its method, and at last enters into its positive phase.