“I designate by this word “milieu,” says Comte, in excusing himself for the new meaning which he gives it, “not only the fluid in which the organism is immersed, but, in general, the totality of external circumstances of any kind whatever necessary to the existence of each determined organism.”[62] Properly speaking then, Comte does not reject the doctrine of final causes; he only transforms it. He had declared this himself in his _opuscule_ in 1822. “The doctrine of final causes has been converted by the physiologists into the principle of the conditions of existence.” Positive philosophy appropriates, “with the understanding of a suitable change,” the general ideas primitively invented by the theological and metaphysical philosophies. As the positive notion of the mathematical laws of phenomena arose out of the metaphysical conceptions of the Pythagoricians concerning the properties of numbers, so the scientific principle of the conditions of existence springs from the hypothesis of final causes.[63] An example will allow us to realise this transformation in the act. The stability of the solar system renders the existence of living species on the earth possible. A good example of finality it would seem. Nevertheless this stability is simply a necessary consequence, according to the mechanical laws of the world, of some circumstances characteristic of our system: extreme smallness of the planetary masses in comparison to the central mass, small eccentricity of their orbits, slight mutual inclination of their planes, etc. Since, in fact, we exist it must be that the system of which we form a part is arranged so as to allow of this existence.” The so-called final cause would then reduce itself here, as on all analogous occasions, to this childish remark: the only stars inhabited are those which are habitable. In a word, we return to the principle of the conditions of existence, which is the true positive transformation of the doctrine of final causes, and whose bearings and fertility are far superior.”[64] In order to give the formula of this principle, we must have recourse to the general distinction established by de Blainville between the static point of view and the dynamic point of view. Every active being, and in particular every living being, can be analysed from these two points of view. The static analysis considers its elements in their relations of simultaneous connexions. The dynamic analysis discovers the laws of their joint evolution. The first is the share of the anatomist, the second that of the physiologist. Now it is clear that these two analyses are complementary to one another, and are even separately unintelligible. For instance, the anatomist is constantly guided by physiological considerations. Conversely, without anatomical knowledge there is no positive physiology.