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.

                                 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.

                                 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
consists of a term only, try it as they may, clearly do not render the
definition of the thing in question, because a definition is always
a phrase of a certain kind. One may, however, use the word
'definitory' also of such a remark as 'The "becoming" is "beautiful"',
and likewise also of the question, 'Are sensation and knowledge the
same or different?', for argument about definitions is mostly
concerned with questions of sameness and difference. In a word we
may call 'definitory' everything that falls under the same branch of
inquiry as definitions; and that all the above-mentioned examples
are of this character is clear on the face of them. For if we are able
to argue that two things are the same or are different, we shall be
well supplied by the same turn of argument with lines of attack upon
their definitions as well: for when we have shown that they are not
the same we shall have demolished the definition. Observe, please,
that the converse of this last statement does not hold: for to show
that they are the same is not enough to establish a definition. To
show, however, that they are not the same is enough of itself to
overthrow it.

  A 'property' is a predicate which does not indicate the essence of a
thing, but yet belongs to that thing alone, and is predicated
convertibly of it. Thus it is a property of man to-be-capable of
learning grammar: for if A be a man, then he is capable of learning
grammar, and if he be capable of learning grammar, he is a man. For no
one calls anything a 'property' which may possibly belong to something
else, e.g. 'sleep' in the case of man, even though at a certain time
it may happen to belong to him alone. That is to say, if any such
thing were actually to be called a property, it will be called not a
'property' absolutely, but a 'temporary' or a 'relative' property: for
'being on the right hand side' is a temporary property, while
'two-footed' is in point of fact ascribed as a property in certain
relations; e.g. it is a property of man relatively to a horse and a
dog. That nothing which may belong to anything else than A is a
convertible predicate of A is clear: for it does not necessarily
follow that if something is asleep it is a man.

  A 'genus' is what is predicated in the category of essence of a
number of things exhibiting differences in kind. We should treat as
predicates in the category of essence all such things as it would be
appropriate to mention in reply to the question, 'What is the object
before you?'; as, for example, in the case of man, if asked that
question, it is appropriate to say 'He is an animal'. The question,
'Is one thing in the same genus as another or in a different one?'
is also a 'generic' question; for a question of that kind as well
falls under the same branch of inquiry as the genus: for having argued
that 'animal' is the genus of man, and likewise also of ox, we shall
have argued that they are in the same genus; whereas if we show that
it is the genus of the one but not of the other, we shall have
argued that these things are not in the same genus.

  An 'accident' is (i) something which, though it is none of the