The Complete Aristotle (eng.)

Moreover, in using the subordinate term one is bound to use
the other as well: for whoever employs the term ‘virtue’ employs
the term ‘good’, seeing that virtue is a certain kind of good:
likewise, also, whoever employs the term ‘half’ employs the term
‘even’, for to be ‘divided in half’ means to be divided into two,
and two is even.
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5
    Generally speaking, then, one commonplace rule relates to the
failure to frame the expression by means of terms that are prior
and more intelligible: and of this the subdivisions are those
specified above. A second is, see whether, though the object is in
a genus, it has not been placed in a genus. This sort of error is
always found where the essence of the object does not stand first
in the expression, e.g. the definition of ‘body’ as ‘that which has
three dimensions’, or the definition of ‘man’, supposing any one to
give it, as ‘that which knows how to count’: for it is not stated
what it is that has three dimensions, or what it is that knows how
to count: whereas the genus is meant to indicate just this, and is
submitted first of the terms in the definition.
    Moreover, see if, while the term to be defined is used in
relation to many things, he has failed to render it in relation to
all of them; as (e.g.) if he define ‘grammar’ as the ‘knowledge how
to write from dictation’: for he ought also to say that it is a
knowledge how to read as well. For in rendering it as ‘knowledge of
writing’ has no more defined it than by rendering it as ‘knowledge
of reading’: neither in fact has succeeded, but only he who
mentions both these things, since it is impossible that there
should be more than one definition of the same thing. It is only,
however, in some cases that what has been said corresponds to the
actual state of things: in some it does not, e.g. all those terms
which are not used essentially in relation to both things: as
medicine is said to deal with the production of disease and health;
for it is said essentially to do the latter, but the former only by
accident: for it is absolutely alien to medicine to produce
disease. Here, then, the man who renders medicine as relative to
both of these things has not defined it any better than he who
mentions the one only. In fact he has done it perhaps worse, for
any one else besides the doctor is capable of producing
disease.
    Moreover, in a case where the term to be defined is used in
relation to several things, see if he has rendered it as relative
to the worse rather than to the better; for every form of knowledge
and potentiality is generally thought to be relative to the
best.
    Again, if the thing in question be not placed in its own proper
genus, one must examine it according to the elementary rules in
regard to genera, as has been said before.’
    Moreover, see if he uses language which transgresses the genera
of the things he defines, defining, e.g. justice as a ‘state that
produces equality’ or ‘distributes what is equal’: for by defining
it so he passes outside the sphere of virtue, and so by leaving out
the genus of justice he fails to express its essence: for the
essence of a thing must in each case bring in its genus. It is the
same thing if the object be not put into its nearest genus; for the
man who puts it into the nearest one has stated all the higher
genera, seeing that all the higher genera are predicated of the
lower. Either, then, it ought to be put into its nearest genus, or
else to the higher genus all the differentiae ought to be appended
whereby the nearest genus is defined. For then he would not have
left out anything: but would merely have mentioned the subordinate
genus by an expression instead of by name. On the other hand, he
who mentions merely the higher genus by itself, does not state the
subordinate genus as well: in saying ‘plant’ a man does not specify
‘a tree’.
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6
    Again, in regard to the differentiae, we must examine in like
manner whether the differentiae, too, that he has stated be those
of the genus. For if a man has not defined the object by the
differentiae peculiar to it, or has mentioned something such as is
utterly incapable of being a differentia of anything, e.g. ‘animal’
or ‘substance’, clearly he has not defined it at all: for the
aforesaid terms do not differentiate anything at all. Further, we
must see whether the