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.
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