The Complete Aristotle (eng.)

or
possible. For the inquiry will be the same, and the syllogism will
proceed through terms arranged in the same order whether a possible
or a pure proposition is proved. We must find in the case of
possible relations, as well as terms that belong, terms which can
belong though they actually do not: for we have proved that the
syllogism which establishes a possible relation proceeds through
these terms as well. Similarly also with the other modes of
predication.
    It is clear then from what has been said not only that all
syllogisms can be formed in this way, but also that they cannot be
formed in any other. For every syllogism has been proved to be
formed through one of the aforementioned figures, and these cannot
be composed through other terms than the consequents and
antecedents of the terms in question: for from these we obtain the
premisses and find the middle term. Consequently a syllogism cannot
be formed by means of other terms.
30
    The method is the same in all cases, in philosophy, in any art
or study. We must look for the attributes and the subjects of both
our terms, and we must supply ourselves with as many of these as
possible, and consider them by means of the three terms, refuting
statements in one way, confirming them in another, in the pursuit
of truth starting from premisses in which the arrangement of the
terms is in accordance with truth, while if we look for dialectical
syllogisms we must start from probable premisses. The principles of
syllogisms have been stated in general terms, both how they are
characterized and how we must hunt for them, so as not to look to
everything that is said about the terms of the problem or to the
same points whether we are confirming or refuting, or again whether
we are confirming of all or of some, and whether we are refuting of
all or some. we must look to fewer points and they must be
definite. We have also stated how we must select with reference to
everything that is, e.g. about good or knowledge. But in each
science the principles which are peculiar are the most numerous.
Consequently it is the business of experience to give the
principles which belong to each subject. I mean for example that
astronomical experience supplies the principles of astronomical
science: for once the phenomena were adequately apprehended, the
demonstrations of astronomy were discovered. Similarly with any
other art or science. Consequently, if the attributes of the thing
are apprehended, our business will then be to exhibit readily the
demonstrations. For if none of the true attributes of things had
been omitted in the historical survey, we should be able to
discover the proof and demonstrate everything which admitted of
proof, and to make that clear, whose nature does not admit of
proof.
    In general then we have explained fairly well how we must select
premisses: we have discussed the matter accurately in the treatise
concerning dialectic.
31
    It is easy to see that division into classes is a small part of
the method we have described: for division is, so to speak, a weak
syllogism; for what it ought to prove, it begs, and it always
establishes something more general than the attribute in question.
First, this very point had escaped all those who used the method of
division; and they attempted to persuade men that it was possible
to make a demonstration of substance and essence. Consequently they
did not understand what it is possible to prove syllogistically by
division, nor did they understand that it was possible to prove
syllogistically in the manner we have described. In demonstrations,
when there is a need to prove a positive statement, the middle term
through which the syllogism is formed must always be inferior to
and not comprehend the first of the extremes. But division has a
contrary intention: for it takes the universal as middle. Let
animal be the term signified by A, mortal by B, and immortal by C,
and let man, whose definition is to be got, be signified by D. The
man who divides assumes that every animal is either mortal or
immortal: i.e. whatever is A is all either B or C. Again, always
dividing, he lays it down that man is an animal, so he assumes A of
D as belonging to it. Now the true conclusion is that every D is
either B or C, consequently man must be either mortal or immortal,
but it is not necessary that man should be a mortal animal-this is
begged: and this is what ought to have been proved syllogistically.
And again, taking A as mortal animal, B as footed,