The Complete Aristotle (eng.)

genus and
differentia must of necessity be known as well (for any one who
knows what a man is knows also what ‘animal’ and ‘walking’ are),
whereas if the genus or the differentia be known it does not follow
of necessity that the species is known as well: thus the species is
less intelligible. Moreover, those who say that such definitions,
viz. those which proceed from what is intelligible to this, that,
or the other man, are really and truly definitions, will have to
say that there are several definitions of one and the same thing.
For, as it happens, different things are more intelligible to
different people, not the same things to all; and so a different
definition would have to be rendered to each several person, if the
definition is to be constructed from what is more intelligible to
particular individuals. Moreover, to the same people different
things are more intelligible at different times; first of all the
objects of sense; then, as they become more sharpwitted, the
converse; so that those who hold that a definition ought to be
rendered through what is more intelligible to particular
individuals would not have to render the same definition at all
times even to the same person. It is clear, then, that the right
way to define is not through terms of that kind, but through what
is absolutely more intelligible: for only in this way could the
definition come always to be one and the same. Perhaps, also, what
is absolutely intelligible is what is intelligible, not to all, but
to those who are in a sound state of understanding, just as what is
absolutely healthy is what is healthy to those in a sound state of
body. All such points as this ought to be made very precise, and
made use of in the course of discussion as occasion requires. The
demolition of a definition will most surely win a general approval
if the definer happens to have framed his expression neither from
what is absolutely more intelligible nor yet from what is so to
us.
    One form, then, of the failure to work through more intelligible
terms is the exhibition of the prior through the posterior, as we
remarked before.’ Another form occurs if we find that the
definition has been rendered of what is at rest and definite
through what is indefinite and in motion: for what is still and
definite is prior to what is indefinite and in motion.
    Of the failure to use terms that are prior there are three
forms:
    (1) The first is when an opposite has been defined through its
opposite, e.g.i. good through evil: for opposites are always
simultaneous by nature. Some people think, also, that both are
objects of the same science, so that the one is not even more
intelligible than the other. One must, however, observe that it is
perhaps not possible to define some things in any other way, e.g.
the double without the half, and all the terms that are essentially
relative: for in all such cases the essential being is the same as
a certain relation to something, so that it is impossible to
understand the one term without the other, and accordingly in the
definition of the one the other too must be embraced. One ought to
learn up all such points as these, and use them as occasion may
seem to require.
    (2) Another is-if he has used the term defined itself. This
passes unobserved when the actual name of the object is not used,
e.g. supposing any one had defined the sun as a star that appears
by day’. For in bringing in ‘day’ he brings in the sun. To detect
errors of this sort, exchange the word for its definition, e.g. the
definition of ‘day’ as the ‘passage of the sun over the earth’.
Clearly, whoever has said ‘the passage of the sun over the earth’
has said ‘the sun’, so that in bringing in the ‘day’ he has brought
in the sun.
    (3) Again, see if he has defined one coordinate member of a
division by another, e.g. ‘an odd number’ as ‘that which is greater
by one than an even number’. For the co-ordinate members of a
division that are derived from the same genus are simultaneous by
nature and ‘odd’ and ‘even’ are such terms: for both are
differentiae of number.
    Likewise also, see if he has defined a superior through a
subordinate term, e.g. ‘An “even number” is “a number divisible
into halves”’, or ‘”the good” is a “state of virtue” ‘. For ‘half’
is derived from ‘two’, and ‘two’ is an even number: virtue also is
a kind of good, so that the latter terms are subordinate to the
former.