The Complete Aristotle (eng.)

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‘capacity’ have more claim than ‘virtue’ to be the genus of
self-control, and virtue be the genus, so also is capacity. The
same observations will apply also in the case of the species. For
instance, supposing A and B to have a like claim to be a species of
the genus in question, then if the one be a species, so also is the
other: and if that which is less generally thought to be so be a
species, so also is that which is more generally thought to be
so.
    Moreover, to establish a view, you should look and see if the
genus is predicated in the category of essence of those things of
which it has been rendered as the genus, supposing the species
rendered to be not one single species but several different ones:
for then clearly it will be the genus. If, on the other, the
species rendered be single, look and see whether the genus be
predicated in the category of essence of other species as well: for
then, again, the result will be that it is predicated of several
different species.
    Since some people think that the differentia, too, is a
predicate of the various species in the category of essence, you
should distinguish the genus from the differentia by employing the
aforesaid elementary principles-(a) that the genus has a wider
denotation than the differentia; (b) that in rendering the essence
of a thing it is more fitting to state the genus than the
differentia: for any one who says that ‘man’ is an ‘animal’ shows
what man is better than he who describes him as ‘walking’; also (c)
that the differentia always signifies a quality of the genus,
whereas the genus does not do this of the differentia: for he who
says ‘walking’ describes an animal of a certain quality, whereas he
who says ‘animal’ describes an animal of a certain quality, whereas
he who says ‘animal’ does not describe a walking thing of a certain
quality.
    The differentia, then, should be distinguished from the genus in
this manner. Now seeing it is generally held that if what is
musical, in being musical, possesses knowledge in some respect,
then also ‘music’ is a particular kind of ‘knowledge’; and also
that if what walks is moved in walking, then ‘walking’ is a
particular kind of ‘movement’; you should therefore examine in the
aforesaid manner any genus in which you want to establish the
existence of something; e.g. if you wish to prove that ‘knowledge’
is a form of ‘conviction’, see whether the knower in knowing is
convinced: for then clearly knowledge would be a particular kind of
conviction. You should proceed in the same way also in regard to
the other cases of this kind.
    Moreover, seeing that it is difficult to distinguish whatever
always follows along with a thing, and is not convertible with it,
from its genus, if A follows B universally, whereas B does not
follow A universally-as e.g. ‘rest’ always follows a ‘calm’ and
‘divisibility’ follows ‘number’, but not conversely (for the
divisible is not always a number, nor rest a calm)-you may yourself
assume in your treatment of them that the one which always follows
is the genus, whenever the other is not convertible with it: if, on
the other hand, some one else puts forward the proposition, do not
accept it universally. An objection to it is that ‘not-being’
always follows what is ‘coming to be’ (for what is coming to be is
not) and is not convertible with it (for what is not is not always
coming to be), and that still ‘not-being’ is not the genus of
‘coming to be’: for ‘not-being’ has not any species at all.
Questions, then, in regard to Genus should be investigated in the
ways described.

Topics, Book V
    Translated by W. A. Pickard-Cambridge
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    div id="book4" class="book" title="Book IV">
    1
    The question whether the attribute stated is or is not a
property, should be examined by the following methods:
    Any ‘property’ rendered is always either essential and permanent
or relative and temporary: e.g. it is an ‘essential property’ of
man to be ‘by nature a civilized animal’: a ‘relative property’ is
one like that of the soul in relation to the body, viz. that the
one is fitted to command, and the other to obey: a ‘permanent
property’ is one like the property which belongs to God, of being
an ‘immortal living being’: a ‘temporary property’ is one like the
property which belongs to any particular man of walking in the
gymnasium.
    [The rendering of a property ‘relatively’ gives rise