The Complete Aristotle (eng.)

these
therefore are those which it is most important to master and to
have ready to hand: for they are the most useful on the greatest
number of occasions. Of the rest, too, the most important are those
of most general application: for these are the most effective, e.g.
that you should examine the individual cases, and then look to see
in the case of their various species whether the definition
applies. For the species is synonymous with its individuals. This
sort of inquiry is of service against those who assume the
existence of Ideas, as has been said before.’ Moreover see if a man
has used a term metaphorically, or predicated it of itself as
though it were something different. So too if any other of the
commonplace rules is of general application and effective, it
should be employed.
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5
    That it is more difficult to establish than to overthrow a
definition, is obvious from considerations presently to be urged.
For to see for oneself, and to secure from those whom one is
questioning, an admission of premisses of this sort is no simple
matter, e.g. that of the elements of the definition rendered the
one is genus and the other differentia, and that only the genus and
differentiae are predicated in the category of essence. Yet without
these premisses it is impossible to reason to a definition; for if
any other things as well are predicated of the thing in the
category of essence, there is no telling whether the formula stated
or some other one is its definition, for a definition is an
expression indicating the essence of a thing. The point is clear
also from the following: It is easier to draw one conclusion than
many. Now in demolishing a definition it is sufficient to argue
against one point only (for if we have overthrown any single point
whatsoever, we shall have demolished the definition); whereas in
establishing a definition, one is bound to bring people to the view
that everything contained in the definition is attributable.
Moreover, in establishing a case, the reasoning brought forward
must be universal: for the definition put forward must be
predicated of everything of which the term is predicated, and must
moreover be convertible, if the definition rendered is to be
peculiar to the subject. In overthrowing a view, on the other hand,
there is no longer any necessity to show one’s point universally:
for it is enough to show that the formula is untrue of any one of
the things embraced under the term.
    Further, even supposing it should be necessary to overthrow
something by a universal proposition, not even so is there any need
to prove the converse of the proposition in the process of
overthrowing the definition. For merely to show that the definition
fails to be predicated of every one of the things of which the term
is predicated, is enough to overthrow it universally: and there is
no need to prove the converse of this in order to show that the
term is predicated of things of which the expression is not
predicated. Moreover, even if it applies to everything embraced
under the term, but not to it alone, the definition is thereby
demolished.
    The case stands likewise in regard to the property and genus of
a term also. For in both cases it is easier to overthrow than to
establish. As regards the property this is clear from what has been
said: for as a rule the property is rendered in a complex phrase,
so that to overthrow it, it is only necessary to demolish one of
the terms used, whereas to establish it is necessary to reason to
them all. Then, too, nearly all the other rules that apply to the
definition will apply also to the property of a thing. For in
establishing a property one has to show that it is true of
everything included under the term in question, whereas to
overthrow one it is enough to show in a single case only that it
fails to belong: further, even if it belongs to everything falling
under the term, but not to that only, it is overthrown in this case
as well, as was explained in the case of the definition. In regard
to the genus, it is clear that you are bound to establish it in one
way only, viz. by showing that it belongs in every case, while of
overthrowing it there are two ways: for if it has been shown that
it belongs either never or not in a certain case, the original
statement has been demolished. Moreover, in establishing a genus it
is not enough to show that it belongs, but also that it belongs as
genus has to be shown; whereas in