The Complete Aristotle (eng.)

does the whole
definition define the whole complex. Suppose, e.g. that some one
has defined a ‘finite straight line’ as ‘the limit of a finite
plane, such that its centre is in a line with its extremes’; if now
the definition of a finite line’ be the ‘limit of a finite plane’,
the rest (viz. ‘such that its centre is in a line with its
extremes’) ought to be a definition of straight’. But an infinite
straight line has neither centre nor extremes and yet is straight
so that this remainder does not define the remainder of the
term.
    Moreover, if the term defined be a compound notion, see if the
definition rendered be equimembral with the term defined. A
definition is said to be equimembral with the term defined when the
number of the elements compounded in the latter is the same as the
number of nouns and verbs in the definition. For the exchange in
such cases is bound to be merely one of term for term, in the case
of some if not of all, seeing that there are no more terms used now
than formerly; whereas in a definition terms ought to be rendered
by phrases, if possible in every case, or if not, in the majority.
For at that rate, simple objects too could be defined by merely
calling them by a different name, e.g. ‘cloak’ instead of
‘doublet’.
    The mistake is even worse, if actually a less well known term be
substituted, e.g. ‘pellucid mortal’ for ‘white man’: for it is no
definition, and moreover is less intelligible when put in that
form.
    Look and see also whether, in the exchange of words, the sense
fails still to be the same. Take, for instance, the explanation of
‘speculative knowledge’ as ‘speculative conception’: for conception
is not the same as knowledge-as it certainly ought to be if the
whole is to be the same too: for though the word ‘speculative’ is
common to both expressions, yet the remainder is different.
    Moreover, see if in replacing one of the terms by something else
he has exchanged the genus and not the differentia, as in the
example just given: for ‘speculative’ is a less familiar term than
knowledge; for the one is the genus and the other the differentia,
and the genus is always the most familiar term of all; so that it
is not this, but the differentia, that ought to have been changed,
seeing that it is the less familiar. It might be held that this
criticism is ridiculous: because there is no reason why the most
familiar term should not describe the differentia, and not the
genus; in which case, clearly, the term to be altered would also be
that denoting the genus and not the differentia. If, however, a man
is substituting for a term not merely another term but a phrase,
clearly it is of the differentia rather than of the genus that a
definition should be rendered, seeing that the object of rendering
the definition is to make the subject familiar; for the differentia
is less familiar than the genus.
    If he has rendered the definition of the differentia, see
whether the definition rendered is common to it and something else
as well: e.g. whenever he says that an odd number is a ‘number with
a middle’, further definition is required of how it has a middle:
for the word ‘number’ is common to both expressions, and it is the
word ‘odd’ for which the phrase has been substituted. Now both a
line and a body have a middle, yet they are not ‘odd’; so that this
could not be a definition of ‘odd’. If, on the other hand, the
phrase ‘with a middle’ be used in several senses, the sense here
intended requires to be defined. So that this will either discredit
the definition or prove that it is no definition at all.
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    Again, see if the term of which he renders the definition is a
reality, whereas what is contained in the definition is not, e.g.
Suppose ‘white’ to be defined as ‘colour mingled with fire’: for
what is bodiless cannot be mingled with body, so that ‘colour’
‘mingled with fire’ could not exist, whereas ‘white’ does
exist.
    Moreover, those who in the case of relative terms do not
distinguish to what the object is related, but have described it
only so as to include it among too large a number of things, are
wrong either wholly or in part; e.g. suppose some one to have
defined ‘medicine’ as a science of Reality’. For if medicine be not
a science of anything that is real, the definition is clearly
altogether false; while if it be a science of some real