animal falls. Likewise in regard to each of the rest, both of those genera which are external to animal, and of those which are contained under it, as of bird, is that into which every bird falls, and of fish that into which every fish falls. Thus proceeding we may know that nothing is omitted, but otherwise we must omit something, and not know it. It is not at all necessary that he who defines and divides, should know all things that subsist, though some say it is impossible to know the differences of each thing without knowing each; but it is impossible to know each thing without differences, for that from which this does not differ, is the same with this, but that from which it differs is something else than this. In the first place then this is false, for it is not something else according to every difference, since there are many differences in things which are the same in species, yet not according to substance, nor per se. Next, when any one assumes opposites, and difference, and that every thing falls into this or that, and assumes also that the question is in one part of the two, and knows this, it is of no consequence whether he knows or does not those other things of which the differences are predicated. For it is evident that thus proceeding, if he should arrive at those of which there is no longer a difference, he will obtain the definition of the substance; but that every thing will fall into division, if there should be opposites of which there is no medium, is not a postulate, since every thing must necessarily be in one of them, if indeed it will be the difference of it.
In order to frame definition by divisions, we must attend to three things, viz. to assume the things predicated in respect of what a thing is; to arrange these, which shall be first or second; and that these are all. Now the first of
