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is useful as a training is obvious on the face of it. The possession
of a plan of inquiry will enable us more easily to argue about the
subject proposed. For purposes of casual encounters, it is useful
because when we have counted up the opinions held by most people, we
shall meet them on the ground not of other people's convictions but of
their own, while we shift the ground of any argument that they appear
to us to state unsoundly. For the study of the philosophical sciences
it is useful, because the ability to raise searching difficulties on
both sides of a subject will make us detect more easily the truth and
error about the several points that arise. It has a further use in
relation to the ultimate bases of the principles used in the several
sciences. For it is impossible to discuss them at all from the
principles proper to the particular science in hand, seeing that the
principles are the prius of everything else: it is through the
opinions generally held on the particular points that these have to be
discussed, and this task belongs properly, or most appropriately, to
dialectic: for dialectic is a process of criticism wherein lies the
path to the principles of all inquiries.
Part 3
We shall be in perfect possession of the way to proceed when we are in
a position like that which we occupy in regard to rhetoric and
medicine and faculties of that kind: this means the doing of that
which we choose with the materials that are available. For it is not
every method that the rhetorician will employ to persuade, or the
doctor to heal; still, if he omits none of the available means, we
shall say that his grasp of the science is adequate.
Part 4
First, then, we must see of what parts our inquiry consists. Now if we
were to grasp (a) with reference to how many, and what kind of, things
arguments take place, and with what materials they start, and (h) how
we are to become well supplied with these, we should have sufficiently
won our goal. Now the materials with which arguments start are equal
in number, and are identical, with the subjects on which reasonings
take place. For arguments start with 'propositions', while the
subjects on which reasonings take place are 'problems'. Now every
proposition and every problem indicates either a genus or a
peculiarity or an accident-for the differentia too, applying as it
does to a class (or genus), should be ranked together with the genus.
Since, however, of what is peculiar to anything part signifies its
essence, while part does not, let us divide the 'peculiar' into both
the aforesaid parts, and call that part which indicates the essence a
'definition', while of the remainder let us adopt the terminology
which is generally current about these things, and speak of it as a
'property'. What we have said, then, makes it clear that according to
our present division, the elements turn out to be four, all told,
namely either property or definition or genus or accident. Do not let
any one suppose us to mean that each of these enunciated by itself
constitutes a proposition or problem, but only that it is from these
that both problems and propositions are formed. The difference between
a problem and a proposition is a difference in the turn of the phrase.
For if it be put in this way, "'An animal that walks on two feet" is
the definition of man, is it not?' or '"Animal" is the genus of man,
is it not?' the result is a proposition: but if thus, 'Is "an animal
that walks on two feet" a definition of man or no?' [or 'Is "animal"
his genus or no?'] the result is a problem. Similarly too in other
cases. Naturally, then, problems and propositions are equal in number:
for out of every proposition you will make a problem if you change the
turn of the phrase.
Part 5
We must now say what are 'definition', 'property', 'genus', and
'accident'. A 'definition' is a phrase signifying a thing's essence.
It is rendered in the form either of a phrase in lieu of a term, or of
a phrase in lieu of another phrase; for it is sometimes possible to
define the meaning of a phrase as well. People whose rendering