(6) Apart from the great difficulty of stating the case truly with
regard to these matters, it is very hard to say, with regard to the
first principles, whether it is the genera that should be taken as
elements and principles, or rather the primary constituents of a
thing; e.g. it is the primary parts of which articulate sounds consist
that are thought to be elements and principles of articulate sound,
not the common genus-articulate sound; and we give the name of
"elements" to those geometrical propositions, the proofs of which
are implied in the proofs of the others, either of all or of most.
Further, both those who say there are several elements of corporeal
things and those who say there is one, say the parts of which bodies
are compounded and consist are principles; e.g. Empedocles says fire
and water and the rest are the constituent elements of things, but
does not describe these as genera of existing things. Besides this, if
we want to examine the nature of anything else, we examine the parts
of which, e.g. a bed consists and how they are put together, and
then we know its nature.

    To judge from these arguments, then, the principles of things
would not be the genera; but if we know each thing by its
definition, and the genera are the principles or starting-points of
definitions, the genera must also be the principles of definable
things. And if to get the knowledge of the species according to
which things are named is to get the knowledge of things, the genera
are at least starting-points of the species. And some also of those
who say unity or being, or the great and the small, are elements of
things, seem to treat them as genera.

    But, again, it is not possible to describe the principles in
both ways. For the formula of the essence is one; but definition by
genera will be different from that which states the constituent
parts of a thing.

    (7) Besides this, even if the genera are in the highest degree
principles, should one regard the first of the genera as principles,
or those which are predicated directly of the individuals? This also
admits of dispute. For if the universals are always more of the nature
of principles, evidently the uppermost of the genera are the
principles; for these are predicated of all things. There will,
then, be as many principles of things as there are primary genera,
so that both being and unity will be principles and substances; for
these are most of all predicated of all existing things. But it is not
possible that either unity or being should be a single genus of
things; for the differentiae of any genus must each of them both
have being and be one, but it is not possible for the genus taken
apart from its species (any more than for the species of the genus) to
be predicated of its proper differentiae; so that if unity or being is
a genus, no differentia will either have being or be one. But if unity
and being are not genera, neither will they be principles, if the
genera are the principles. Again, the intermediate kinds, in whose
nature the differentiae are included, will on this theory be genera,
down to the indivisible species; but as it is, some are thought to
be genera and others are not thought to be so. Besides this, the
differentiae are principles even more than the genera; and if these
also are principles, there comes to be practically an infinite
number of principles, especially if we suppose the highest genus to be
a principle.-But again, if unity is more of the nature of a principle,
and the indivisible is one, and everything indivisible is so either in
quantity or in species, and that which is so in species is the
prior, and genera are divisible into species for man is not the
genus of individual men), that which is predicated directly of the
individuals will have more unity.-Further, in the case of things in
which the distinction of prior and posterior is present, that which is
predicable of these things cannot be something apart from them (e.g.
if two is the first of numbers, there will not be a Number apart
from the kinds of numbers; and similarly there will not be a Figure
apart from the kinds of figures; and if the genera of these things
do not exist apart from the species, the genera of other things will
scarcely do so; for genera of these things are thought to exist if any
do). But among the individuals one is not prior and another posterior.
Further, where one thing is better and another worse, the better is
always prior; so that of these also no genus can exist. From these
considerations, then, the species predicated of individuals seem to be
principles rather than the genera. But again, it is not easy to say in
what sense these are to be taken as principles. For the principle or
cause must exist alongside of the things of which it is the principle,
and must be capable of existing in separation from them; but for
what reason should we suppose any such thing to exist alongside of the
individual, except that it is predicated universally and of all? But
if this is the reason, the things that are more universal must be
supposed to be more of the nature of principles; so that the highest
genera would be the principles.