The medieval account of physical nature had been dominated by what we may call common-sense biology. Biology, indeed, is the science of the living, and the medievals were no more inclined than we are to endow all physical bodies with life. What they did do was to take living bodies as typical, and to treat other bodies as imperfectly analogous to them. Such an approach was _a priori_ reasonable enough. For we may be expected to know best the physical being closest to our own; and we, at any rate, are alive. Why not argue from the better known to the less known, from the nearer to the more remote, interpreting other things by the formula of our own being, and allowing whatever discount is necessary for their degree of unlikeness to us? Common-sense biology reasons as follows. In a living body there is a certain pattern of organized parts, a certain rhythm of successive motions, and a certain range of characteristic activities. The pattern, the sheer anatomy, is basic; but it cannot long continue to exist (outside a refrigerator) without accompanying vital rhythms in heart, respiration and digestion. Nor do these perform their parts without the intermittent support of variable but still characteristic activities: dogs not only breathe and digest, they run about, hunt their food, look for mates, bark at cats, and so on. The anatomical pattern, the vital rhythm, and the characteristic acts together express dogginess; they reveal the specific form of the dog. They _reveal_ it; exactly what the specific form _consisted in_ was the subject of much medieval speculation. It need not concern us here. Taking the form of the species for granted, common-sense biology proceeds to ask how it comes to be in a given instance, say in the dog Toby. [15] Before this dog was born or thought of, his form or species was displayed in each of his parents. And now it looks as though the form of dog had detached itself from them through the generative act, and set up anew on its own account. How does it do that? By getting hold of some materials in which to express itself. At first it takes them from the body of the mother, afterwards it collects them from a wider environment, and what the dog eats becomes the dog.