Thus, in dealing with Biology the positive method undergoes a veritable inversion. In the preceding sciences, the last degree of composition is forbidden us: we never succeed in uniting the whole of the inorganic world into a single synthesis. In biology, on the contrary, sums of phenomena are given; but it is the last degree of simplicity which escapes us. We have to start from those sums of phenomena, and biology must in this way assume a synthetic character. In it the analysis of phenomena will be as minute as possible; but the analytical operations will always be more or less directly subordinated to the leading idea of the vital _consensus_.[155] II. Like the other fundamental sciences Biology must be abstract, that is to say it must not bear upon individual beings, but upon phenomena. It is thus distinct from zoology and botany which are concrete sciences. In its widest generality it is defined through the constant correspondence between the anatomical and the physiological point of view. Its object is to constantly unite them to one another. In reality these two points of view are the two aspects of a single problem. It is owing to historical reasons that, during a certain time, these two sciences appeared to develop independently of one another. Physiology remained attached to the metaphysical methods, that is to say, to unverifiable hypotheses and to principles which went beyond experience, while anatomists already made use of the positive method. But to-day, the two sciences being equally positive, “their opposition is reduced to that which subsists between the static and the dynamic points of view.”[156] Another element which should enter into the more general definition of biology, although it has sometimes been neglected, is the consideration of the _milieu_. The relation between the organism and its _milieu_ is no less essential to life than the relation of the organ to the function. Life supposes not only that the being should be organised in a certain way, but also that a certain number of external circumstances should sustain this organisation, and should be compatible with its activity. Living beings are thus dependent upon their _milieu_, and this dependence grows as we rise in the organic series. The system of the conditions of existence becomes all the more complex as the functions develop and become more varied. Inferior organisms are subject to less numerous external conditions; but, says Comte, a little variation in one of these conditions suffices to make them perish. The superior organisms stand a variation of this kind better. But, in return, the number of conditions upon which they depend is far greater. The study of _milieux_ in their relations to organisms, a study which is hardly outlined, undoubtedly has many discoveries in store for the future.[157] Here is an order of problems of which Lamarck probably suggested the idea to Comte, and upon which Darwin’s genius will work.