The Complete Aristotle (eng.)

language of a definition ought to be the
very clearest possible, seeing that the whole purpose of rendering
it is to make something known); (secondly, if the expression used
be longer than is necessary: for all additional matter in a
definition is superfluous. Again, each of the aforesaid branches is
divided into a number of others.
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    One commonplace rule, then, in regard to obscurity is, See if
the meaning intended by the definition involves an ambiguity with
any other, e.g. ‘Becoming is a passage into being’, or ‘Health is
the balance of hot and cold elements’. Here ‘passage’ and ‘balance’
are ambiguous terms: it is accordingly not clear which of the
several possible senses of the term he intends to convey. Likewise
also, if the term defined be used in different senses and he has
spoken without distinguishing between them: for then it is not
clear to which of them the definition rendered applies, and one can
then bring a captious objection on the ground that the definition
does not apply to all the things whose definition he has rendered:
and this kind of thing is particularly easy in the case where the
definer does not see the ambiguity of his terms. Or, again, the
questioner may himself distinguish the various senses of the term
rendered in the definition, and then institute his argument against
each: for if the expression used be not adequate to the subject in
any of its senses, it is clear that he cannot have defined it in
any sense aright.
    Another rule is, See if he has used a metaphorical expression,
as, for instance, if he has defined knowledge as ‘unsupplantable’,
or the earth as a ‘nurse’, or temperance as a ‘harmony’. For a
metaphorical expression is always obscure. It is possible, also, to
argue sophistically against the user of a metaphorical expression
as though he had used it in its literal sense: for the definition
stated will not apply to the term defined, e.g. in the case of
temperance: for harmony is always found between notes. Moreover, if
harmony be the genus of temperance, then the same object will occur
in two genera of which neither contains the other: for harmony does
not contain virtue, nor virtue harmony. Again, see if he uses terms
that are unfamiliar, as when Plato describes the eye as
‘brow-shaded’, or a certain spider as poison-fanged’, or the marrow
as ‘boneformed’. For an unusual phrase is always obscure.
    Sometimes a phrase is used neither ambiguously, nor yet
metaphorically, nor yet literally, as when the law is said to be
the ‘measure’ or ‘image’ of the things that are by nature just.
Such phrases are worse than metaphor; for the latter does make its
meaning to some extent clear because of the likeness involved; for
those who use metaphors do so always in view of some likeness:
whereas this kind of phrase makes nothing clear; for there is no
likeness to justify the description ‘measure’ or ‘image’, as
applied to the law, nor is the law ordinarily so called in a
literal sense. So then, if a man says that the law is literally a
‘measure’ or an ‘image’, he speaks falsely: for an image is
something produced by imitation, and this is not found in the case
of the law. If, on the other hand, he does not mean the term
literally, it is clear that he has used an unclear expression, and
one that is worse than any sort of metaphorical expression.
    Moreover, see if from the expression used the definition of the
contrary be not clear; for definitions that have been correctly
rendered also indicate their contraries as well. Or, again, see if,
when it is merely stated by itself, it is not evident what it
defines: just as in the works of the old painters, unless there
were an inscription, the figures used to be unrecognizable.
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    If, then, the definition be not clear, you should proceed to
examine on lines such as these. If, on the other hand, he has
phrased the definition redundantly, first of all look and see
whether he has used any attribute that belongs universally, either
to real objects in general, or to all that fall under the same
genus as the object defined: for the mention of this is sure to be
redundant. For the genus ought to divide the object from things in
general, and the differentia from any of the things contained in
the same genus. Now any term that belongs to everything separates
off the given object from absolutely