The Complete Aristotle (eng.)

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definable form, (2) the arrangement of these in the right order,
(3) the omission of no such elements. The first is feasible because
one can establish genus and differentia through the topic of the
genus, just as one can conclude the inherence of an accident
through the topic of the accident. The right order will be achieved
if the right term is assumed as primary, and this will be ensured
if the term selected is predicable of all the others but not all
they of it; since there must be one such term. Having assumed this
we at once proceed in the same way with the lower terms; for our
second term will be the first of the remainder, our third the first
of those which follow the second in a ‘contiguous’ series, since
when the higher term is excluded, that term of the remainder which
is ‘contiguous’ to it will be primary, and so on. Our procedure
makes it clear that no elements in the definable form have been
omitted: we have taken the differentia that comes first in the
order of division, pointing out that animal, e.g. is divisible
exhaustively into A and B, and that the subject accepts one of the
two as its predicate. Next we have taken the differentia of the
whole thus reached, and shown that the whole we finally reach is
not further divisible-i.e. that as soon as we have taken the last
differentia to form the concrete totality, this totality admits of
no division into species. For it is clear that there is no
superfluous addition, since all these terms we have selected are
elements in the definable form; and nothing lacking, since any
omission would have to be a genus or a differentia. Now the primary
term is a genus, and this term taken in conjunction with its
differentiae is a genus: moreover the differentiae are all
included, because there is now no further differentia; if there
were, the final concrete would admit of division into species,
which, we said, is not the case.
    To resume our account of the right method of investigation: We
must start by observing a set of similar-i.e. specifically
identical-individuals, and consider what element they have in
common. We must then apply the same process to another set of
individuals which belong to one species and are generically but not
specifically identical with the former set. When we have
established what the common element is in all members of this
second species, and likewise in members of further species, we
should again consider whether the results established possess any
identity, and persevere until we reach a single formula, since this
will be the definition of the thing. But if we reach not one
formula but two or more, evidently the definiendum cannot be one
thing but must be more than one. I may illustrate my meaning as
follows. If we were inquiring what the essential nature of pride
is, we should examine instances of proud men we know of to see
what, as such, they have in common; e.g. if Alcibiades was proud,
or Achilles and Ajax were proud, we should find on inquiring what
they all had in common, that it was intolerance of insult; it was
this which drove Alcibiades to war, Achilles wrath, and Ajax to
suicide. We should next examine other cases, Lysander, for example,
or Socrates, and then if these have in common indifference alike to
good and ill fortune, I take these two results and inquire what
common element have equanimity amid the vicissitudes of life and
impatience of dishonour. If they have none, there will be two
genera of pride. Besides, every definition is always universal and
commensurate: the physician does not prescribe what is healthy for
a single eye, but for all eyes or for a determinate species of eye.
It is also easier by this method to define the single species than
the universal, and that is why our procedure should be from the
several species to the universal genera-this for the further reason
too that equivocation is less readily detected in genera than in
infimae species. Indeed, perspicuity is essential in definitions,
just as inferential movement is the minimum required in
demonstrations; and we shall attain perspicuity if we can collect
separately the definition of each species through the group of
singulars which we have established e.g. the definition of
similarity not unqualified but restricted to colours and to
figures; the definition of acuteness, but only of sound-and so
proceed to the common universal with a careful avoidance of
equivocation. We may add that if dialectical disputation must not
employ metaphors,