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used, it is possible that the answerer and the questioner are not
directing their minds upon the same thing: whereas when once it has
been made clear how many meanings there are, and also upon which of
them the former directs his mind when he makes his assertion, the
questioner would then look ridiculous if he failed to address his
argument to this. It helps us also both to avoid being misled and to
mislead by false reasoning: for if we know the number of meanings of a
term, we shall certainly never be misled by false reasoning, but shall
know if the questioner fails to address his argument to the same
point; and when we ourselves put the questions we shall be able to
mislead him, if our answerer happens not to know the number of
meanings of our terms. This, however, is not possible in all cases,
but only when of the many senses some are true and others are false.
This manner of argument, however, does not belong properly to
dialectic; dialecticians should therefore by all means beware of this
kind of verbal discussion, unless any one is absolutely unable to
discuss the subject before him in any other way.
The discovery of the differences of things helps us both in reasonings
about sameness and difference, and also in recognizing what any
particular thing is. That it helps us in reasoning about sameness and
difference is clear: for when we have discovered a difference of any
kind whatever between the objects before us, we shall already have
shown that they are not the same: while it helps us in recognizing
what a thing is, because we usually distinguish the expression that is
proper to the essence of each particular thing by means of the
differentiae that are proper to it.
The examination of likeness is useful with a view both to inductive
arguments and to hypothetical reasonings, and also with a view to the
rendering of definitions. It is useful for inductive arguments,
because it is by means of an induction of individuals in cases that
are alike that we claim to bring the universal in evidence: for it is
not easy to do this if we do not know the points of likeness. It is
useful for hypothetical reasonings because it is a general opinion
that among similars what is true of one is true also of the rest. If,
then, with regard to any of them we are well supplied with matter for
a discussion, we shall secure a preliminary admission that however it
is in these cases, so it is also in the case before us: then when we
have shown the former we shall have shown, on the strength of the
hypothesis, the matter before us as well: for we have first made the
hypothesis that however it is in these cases, so it is also in the
case before us, and have then proved the point as regards these cases.
It is useful for the rendering of definitions because, if we are able
to see in one glance what is the same in each individual case of it,
we shall be at no loss into what genus we ought to put the object
before us when we define it: for of the common predicates that which
is most definitely in the category of essence is likely to be the
genus. Likewise, also, in the case of objects widely divergent, the
examination of likeness is useful for purposes of definition, e.g. the
sameness of a calm at sea, and windlessness in the air (each being a
form of rest), and of a point on a line and the unit in number-each
being a starting point. If, then, we render as the genus what is
common to all the cases, we shall get the credit of defining not
inappropriately. Definition-mongers too nearly always render them in
this way: they declare the unit to be the startingpoint of number, and
the point the startingpoint of a line. It is clear, then, that they
place them in that which is common to both as their genus.
The means, then, whereby reasonings are effected, are these: the
commonplace rules, for the observance of which the aforesaid means are
useful, are as follows.

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By Aristotle
Written 350 B.C.E Part 1
Of problems some are universal, others particular. Universal problems