question of geometry with the geometrician; and likewise also in other
cases.

                                11

  A dialectical problem is a subject of inquiry that contributes
either to choice and avoidance, or to truth and knowledge, and that
either by itself, or as a help to the solution of some other such
problem. It must, moreover, be something on which either people hold
no opinion either way, or the masses hold a contrary opinion to the
philosophers, or the philosophers to the masses, or each of them among
themselves. For some problems it is useful to know with a view to
choice or avoidance, e.g. whether pleasure is to be chosen or not,
while some it is useful to know merely with a view to knowledge,
e.g. whether the universe is eternal or not: others, again, are not
useful in and by themselves for either of these purposes, but yet help
us in regard to some such problems; for there are many things which we
do not wish to know in and by themselves, but for the sake of other
things, in order that through them we may come to know something else.
Problems also include questions in regard to which reasonings conflict
(the difficulty then being whether so-and so is so or not, there being
convincing arguments for both views); others also in regard to which
we have no argument because they are so vast, and we find it difficult
to give our reasons, e.g. the question whether the universe is eternal
or no: for into questions of that kind too it is possible to inquire.

  Problems, then, and propositions are to be defined as aforesaid. A
'thesis' is a supposition of some eminent philosopher that conflicts
with the general opinion; e.g. the view that contradiction is
impossible, as Antisthenes said; or the view of Heraclitus that all
things are in motion; or that Being is one, as Melissus says: for to
take notice when any ordinary person expresses views contrary to men's
usual opinions would be silly. Or it may be a view about which we have
a reasoned theory contrary to men's usual opinions, e.g. the view
maintained by the sophists that what is need not in every case
either have come to be or be eternal: for a musician who is a
grammarian 'is' so without ever having 'come to be' so, or being so
eternally. For even if a man does not accept this view, he might do so
on the ground that it is reasonable.

  Now a 'thesis' also is a problem, though a problem is not always a
thesis, inasmuch as some problems are such that we have no opinion
about them either way. That a thesis, however, also forms a problem,
is clear: for it follows of necessity from what has been said that
either the mass of men disagree with the philosophers about the
thesis, or that the one or the other class disagree among
themselves, seeing that the thesis is a supposition in conflict with
general opinion. Practically all dialectical problems indeed are now
called 'theses'. But it should make no difference whichever
description is used; for our object in thus distinguishing them has
not been to create a terminology, but to recognize what differences
happen to be found between them.

  Not every problem, nor every thesis, should be examined, but only
one which might puzzle one of those who need argument, not
punishment or perception. For people who are puzzled to know whether
one ought to honour the gods and love one's parents or not need
punishment, while those who are puzzled to know whether snow is
white or not need perception. The subjects should not border too
closely upon the sphere of demonstration, nor yet be too far removed
from it: for the former cases admit of no doubt, while the latter
involve difficulties too great for the art of the trainer.

                                12

  Having drawn these definitions, we must distinguish how many species
there are of dialectical arguments. There is on the one hand
Induction, on the other Reasoning. Now what reasoning is has been said
before: induction is a passage from individuals to universals, e.g.
the argument that supposing the skilled pilot is the most effective,
and likewise the skilled charioteer, then in general the skilled man
is the best at his particular task. Induction is the more convincing
and clear: it is more readily learnt by the use of the senses, and
is applicable generally to the mass of men, though reasoning is more
forcible and effective against contradictious people.

                                13

  The classes, then, of things about which, and of things out of
which, arguments are constructed, are to be distinguished in the way
we have said before. The means whereby we are to become well
supplied with reasonings are four: (1) the securing of propositions;
(2) the power to distinguish in how many senses particular
expression is used; (3) the discovery of the differences of things;
(4) the investigation of likeness. The last three, as well, are in a
certain sense propositions: for it is possible to make a proposition
corresponding to each of them, e.g. (1) 'The desirable may mean either
the honourable or the pleasant or the expedient'; and (2) Sensation
differs from knowledge in that the latter may be recovered again after
it has been lost, while the former cannot'; and (3) The relation of
the healthy to health is like that of the vigorous to vigour'. The
first proposition depends upon the use of one term in several
senses, the second upon the differences of things, the third upon
their likenesses.

                                14

  Propositions should be selected in a number of ways corresponding to
the number of distinctions drawn in regard to the proposition: thus
one may first take in hand the opinions held by all or by most men