The Complete Aristotle (eng.)

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absolutely that a thing cannot have as its differentia anything in
respect of which it is subject to alteration: for all things of
that kind, if intensified, destroy its essence. If, then, a man has
rendered any differentia of this kind, he has made a mistake: for
we undergo absolutely no alteration in respect of our
differentiae.
    Again, see if he has failed to render the differentia of a
relative term relatively to something else; for the differentiae of
relative terms are themselves relative, as in the case also of
knowledge. This is classed as speculative, practical and
productive; and each of these denotes a relation: for it speculates
upon something, and produces something and does something.
    Look and see also if the definer renders each relative term
relatively to its natural purpose: for while in some cases the
particular relative term can be used in relation to its natural
purpose only and to nothing else, some can be used in relation to
something else as well. Thus sight can only be used for seeing, but
a strigil can also be used to dip up water. Still, if any one were
to define a strigil as an instrument for dipping water, he has made
a mistake: for that is not its natural function. The definition of
a thing’s natural function is ‘that for which it would be used by
the prudent man, acting as such, and by the science that deals
specially with that thing’.
    Or see if, whenever a term happens to be used in a number of
relations, he has failed to introduce it in its primary relation:
e.g. by defining ‘wisdom’ as the virtue of ‘man’ or of the ‘soul,’
rather than of the ‘reasoning faculty’: for ‘wisdom’ is the virtue
primarily of the reasoning faculty: for it is in virtue of this
that both the man and his soul are said to be wise.
    Moreover, if the thing of which the term defined has been stated
to be an affection or disposition, or whatever it may be, be unable
to admit it, the definer has made a mistake. For every disposition
and every affection is formed naturally in that of which it is an
affection or disposition, as knowledge, too, is formed in the soul,
being a disposition of soul. Sometimes, however, people make bad
mistakes in matters of this sort, e.g. all those who say that
‘sleep’ is a ‘failure of sensation’, or that ‘perplexity’ is a
state of ‘equality between contrary reasonings’, or that ‘pain’ is
a ‘violent disruption of parts that are naturally conjoined’. For
sleep is not an attribute of sensation, whereas it ought to be, if
it is a failure of sensation. Likewise, perplexity is not an
attribute of opposite reasonings, nor pain of parts naturally
conjoined: for then inanimate things will be in pain, since pain
will be present in them. Similar in character, too, is the
definition of ‘health’, say, as a ‘balance of hot and cold
elements’: for then health will be necessarily exhibited by the hot
and cold elements: for balance of anything is an attribute inherent
in those things of which it is the balance, so that health would be
an attribute of them. Moreover, people who define in this way put
effect for cause, or cause for effect. For the disruption of parts
naturally conjoined is not pain, but only a cause of pain: nor
again is a failure of sensation sleep, but the one is the cause of
the other: for either we go to sleep because sensation fails, or
sensation fails because we go to sleep. Likewise also an equality
between contrary reasonings would be generally considered to be a
cause of perplexity: for it is when we reflect on both sides of a
question and find everything alike to be in keeping with either
course that we are perplexed which of the two we are to do.
    Moreover, with regard to all periods of time look and see
whether there be any discrepancy between the differentia and the
thing defined: e.g. supposing the ‘immortal’ to be defined as a
‘living thing immune at present from destruction’. For a living
thing that is immune ‘at present’ from destruction will be immortal
‘at present’. Possibly, indeed, in this case this result does not
follow, owing to the ambiguity of the words ‘immune at present from
destruction’: for it may mean either that the thing has not been
destroyed at present, or that it cannot be destroyed at present, or
that at present it is such that it never can be destroyed.
Whenever, then, we say that a living thing is at present immune
from destruction, we mean that it is at present a