The Complete Aristotle (eng.)

deeds—real or imaginary—that
stand to the credit of those eulogized. On the same principle,
invectives are based on facts of the opposite kind: the orator
looks to see what base deeds—real or imaginary—stand to the
discredit of those he is attacking, such as treachery to the cause
of Hellenic freedom, or the enslavement of their gallant allies
against the barbarians (Aegina, Potidaea, &c.), or any other
misdeeds of this kind that are recorded against them. So, too, in a
court of law: whether we are prosecuting or defending, we must pay
attention to the existing facts of the case. It makes no difference
whether the subject is the Lacedaemonians or the Athenians, a man
or a god; we must do the same thing. Suppose it to be Achilles whom
we are to advise, to praise or blame, to accuse or defend; here too
we must take the facts, real or imaginary; these must be our
material, whether we are to praise or blame him for the noble or
base deeds he has done, to accuse or defend him for his just or
unjust treatment of others, or to advise him about what is or is
not to his interest. The same thing applies to any subject
whatever. Thus, in handling the question whether justice is or is
not a good, we must start with the real facts about justice and
goodness. We see, then, that this is the only way in which any one
ever proves anything, whether his arguments are strictly cogent or
not: not all facts can form his basis, but only those that bear on
the matter in hand: nor, plainly, can proof be effected otherwise
by means of the speech. Consequently, as appears in the Topics, we
must first of all have by us a selection of arguments about
questions that may arise and are suitable for us to handle; and
then we must try to think out arguments of the same type for
special needs as they emerge; not vaguely and indefinitely, but by
keeping our eyes on the actual facts of the subject we have to
speak on, and gathering in as many of them as we can that bear
closely upon it: for the more actual facts we have at our command,
the more easily we prove our case; and the more closely they bear
on the subject, the more they will seem to belong to that speech
only instead of being commonplaces. By ‘commonplaces’ I mean, for
example, eulogy of Achilles because he is a human being or a
demi-god, or because he joined the expedition against Troy: these
things are true of many others, so that this kind of eulogy applies
no better to Achilles than to Diomede. The special facts here
needed are those that are true of Achilles alone; such facts as
that he slew Hector, the bravest of the Trojans, and Cycnus the
invulnerable, who prevented all the Greeks from landing, and again
that he was the youngest man who joined the expedition, and was not
bound by oath to join it, and so on.
    Here, again, we have our first principle of selection of
Enthymemes-that which refers to the lines of argument selected. We
will now consider the various elementary classes of enthymemes. (By
an ‘elementary class’ of enthymeme I mean the same thing as a ‘line
of argument’.) We will begin, as we must begin, by observing that
there are two kinds of enthymemes. One kind proves some affirmative
or negative proposition; the other kind disproves one. The
difference between the two kinds is the same as that between
syllogistic proof and disproof in dialectic. The demonstrative
enthymeme is formed by the conjunction of compatible propositions;
the refutative, by the conjunction of incompatible
propositions.
    We may now be said to have in our hands the lines of argument
for the various special subjects that it is useful or necessary to
handle, having selected the propositions suitable in various cases.
We have, in fact, already ascertained the lines of argument
applicable to enthymemes about good and evil, the noble and the
base, justice and injustice, and also to those about types of
character, emotions, and moral qualities. Let us now lay hold of
certain facts about the whole subject, considered from a different
and more general point of view. In the course of our discussion we
will take note of the distinction between lines of proof and lines
of disproof: and also of those lines of argument used in what seems
to be enthymemes, but are not, since they do not represent valid
syllogisms. Having made all this clear, we will proceed to classify
Objections and Refutations, showing how they can be brought to bear
upon enthymemes.
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    1. One line of positive proof is based