atoms in a given manner.  It belonged neither to the nature of space to be occupied by just those atoms in just those places, nor to the nature of the atoms to be [17] distributed just like that over space; and so in a certain sense the environment of any atom was an accidental environment.  That is, the particular arrangement of the environment was accidental.  The nature of the environment was not accidental at all.  It was proper to the nature of the atom to be in interaction with other atoms over a spatial field, and it never encountered in the fellow-denizens of space any other nature but its own.  It was not subject to the accident of meeting strange natures, nor of becoming suddenly subject to strange or unequal laws of interaction.  All interactions, being with its own kind, were reciprocal and obedient to a single set of calculable laws.

But the medieval philosophy had asserted accidental relations between distinct sorts of natures, the form of living dog and the form of dead matter, for example.  No one could know a priori what effect an accidental relation would produce, and all accidental relations between different pairs of natures were different:  at the most there was analogy between them.  Every different nature had to be separately observed, and when you had observed them all, you could still simply write an inventory of them, you could not hope to rationalize your body of knowledge.  Let us narrow the field and consider what this doctrine allows us to know about the wood of a certain kind of tree.  We shall begin by observing the impressions it makes on our several senses, and we shall attribute to it a substantial form such as naturally to give rise to these impressions, without, perhaps, being so rash as to claim a knowledge of what this substantial form is.  Still we do not know what its capacities of physical action and passion may be.  We shall find them out by observing it in relation to different ‘natures’.  It turns out to be combustible by fire, resistant to water, tractable to the carpenter’s tools, intractable to his digestive organs, harmless to ostriches, nourishing to wood-beetles.  Each of these capacities of the wood is distinct; we cannot relate them intelligibly to one another, nor deduce them from the assumed fundamental ‘woodiness’.

We can now see why ‘substantial forms’ were the betes noires of the seventeenth-century philosophers.  It was because they turned nature into an unmanageable jungle, in which trees, bushes, and parasites of a thousand kinds wildly interlaced.  There was nothing for it, if science was to proceed, but to clear the ground and replant with spruce in rows:  to postulate a single uniform nature, of which there should be a single science.  Now neither probatology nor cynology could hope to be [18] universal—­the world is not all sheep nor all dog:  it would have to be hylology; for the world is, in its spatial aspect, all material.  Let us say, then, that there is one uniform material