substances, others because they are affections of substance, others because they are a process towards substance, or destructions or privations or qualities of substance, or productive or generative of substance, or of things which are relative to substance, or negations of some of these things or of substance itself. It is for this reason that we say even of non-being that it is non-being. As, then, there is one science which deals with all healthy things, the same applies in the other cases also. For not only in the case of things which have one common notion does the investigation belong to one science, but also in the case of things which are related to one common nature; for even these in a sense have one common notion. It is clear then that it is the work of one science also to study all things that are, qua being.—But everywhere science deals chiefly with that which is primary, and on which the other things depend, and in virtue of which they get their names. If, then, this is substance, it is of substances that the philosopher must grasp the principles and the causes.
Now for every single class of things, as there is one perception, so there is one science, as for instance grammar, being one science, investigates all articulate sounds. Therefore to investigate all the species of being qua being, is the work of a science which is generically one, and to investigate the several species is the work of the specific parts of the science.
If, now, being and unity are the same and are one thing in the sense that they are implied in one another as principle and cause are, not in the sense that they are explained by the same definition (though it makes no difference even if we interpret them similarly—in fact this would strengthen our case); for 'one man' and 'existent man' and 'man' are the same thing, and the doubling of the words in 'the man is one and is a man does not give any new meaning (it is clear that his unity is not severed from his humanity either in coming to be or in ceasing to be; and similarly his being is not severed), so that it is obvious that the addition