In general it does not matter whether it
contains many or few terms, nor, therefore, whether it contains few or
two. Of the two one is differentia and the other genus; e.g., in
"two-footed animal" "animal" is genus, and the other term
differentia.If,
then, the genus absolutely does not exist apart from the species which
it includes, or if it exists, but only as matter (for speech is genus
and matter, and the differentiae make the species, i.e. the letters,
out of it), obviously the definition is the formula composed of the
differentiae.But further we must also divide by the
differentia of the differentia. E.g., "having feet" is a differentia
of "animal"; then in turn we must discover the differentia of "animal
having feet" qua "having feet." Accordingly we
should not say that of "that which has feet" one kind is winged and
another wingless, (that is if we are to speak correctly; if we say
this it will be through incapability), but only that one kind is
cloven-footed and another not; because these are differentiae of
"foot," since cloven-footedness is a kind of footedness.And thus we tend always to
progress until we come to the species which contain no differentiae.
At this point there will be just as many species of foot as there are
differentiae, and the kinds of animals having feet will be equal in
number to the differentiae. Then, if this is so, obviously the ultimate differentia will
be the substance and definition of the thing, since we need not state
the same things more than once in definitions, because this is
superfluous.However, it does happen; for when we say "footed two-footed animal"
we have simply said "animal having feet, having two feet." And if we
divide this by its proper division, we shall be stating the same thing
several times, as many times as there are differentiae.If, then, we keep on taking a differentia of a differentia, one of
them, the last, will be the form and the substance. But if we proceed
with reference to accidental qualities—e.g. if we divide
"that which has feet" into white and black—there will be as
many differentiae as there are divisions. It is therefore obvious that
the definition is the formula derived from the differentiae, and
strictly speaking from the last of them.This will be clear if we change the order of
such definitions, e.g. that of man, saying "two-footed footed animal";
for "footed" is superfluous when we have already said "two-footed."
But there is no question of order in the substance; for how are we to
think of one part as posterior and the other prior?With regard, then, to definitions by division, let
this suffice as a preliminary statement of their nature.