Every form, then, will really be the object of a distinct science. The form of the sheep and the form of the dog have much in common, but that merely happens to be so; we cannot depend upon it, or risk inferences from sheep to dog: we must examine each in itself; we shall really need a science of probatology about sheep, and cynology about dogs. Again, the common material nature has its own principles of being and action, so it will need a science of itself, which we may call hylology. Each of these sciences is mistress in her own province; but how many there are, and how puzzlingly they overlap! So long as we remain within the province of a single science, we may be able to think rigorously, everything will be 'tight'. But as soon as we consider border-issues between one province and another, farewell to exactitude: everything will be 'loose'. We can think out hylology till we are blue in the face, but we shall never discover anything about the entry of material elements into higher organizations, or how they behave when they get there. We may form perfect definitions and descriptions of the form of the dog as such, and still derive no rules for telling what elements of matter will enter into the body of a given dog or how they will be placed when they do. All we can be sure of is, that the dog-form will keep itself going in, and by means of, the material it embodies--unless the dog dies. But what happens to the matter in the body of the dog is 'accidental' to the nature of the matter; and the use of this matter, rather than of some other equally suitable, is accidental to the nature of the dog. No account of material events can dispense with accidental relations altogether. We must at least recognize that there are accidental relations between particular things. Accident in the sense of brute fact had to be acknowledged even by the tidiest and most dogmatic atomism of the last century. That atomism must allow it to be accidental, in this sense, that the 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.