he space surrounding any given atom was occupied by other 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.