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.

What, then, is the relation of the assimilated materials to the dog-form which assimilates them?  Before assimilation, they have their own form.  Before the dog eats the leg of mutton, it has the form given to it by its place in the body of a sheep.  What happens to the mutton?  Is it without remainder transubstantiated from sheep into dog?  It loses all its distinctively sheep-like characteristicsm but there may be some more basically material characteristics which it preserves.  They underlay the structure of the mutton, and they continue to underlie the structure of the dog’s flesh which supplants it.  Whatever these characteristics may be, let us call them common material characteristics, and let us say that they belong to or compose a common material nature.

The common material nature has its own way of existing, and perhaps its own principles of physical action.  We may suppose that we know much or that we know little about it.  This one thing at least we know, that it is capable of becoming alternatively either mutton or dog’s flesh.  It is not essential to it to be mutton, or mutton it would always be; nor dog’s flesh, or it would always be dog’s flesh.  It is capable of becoming either, according as it is captured by one or other system of formal organization.  So the voters who are to go to the polls are, by their common nature, Englishmen; they are essentially neither Socialist curs nor Conservative sheep, but intrinsically capable of becoming either, if they become captured by either system of party organization.