Evidently then it belongs to the philosopher, i.e. to him who is
studying the nature of all substance, to inquire also into the
principles of syllogism. But he who knows best about each genus must
be able to state the most certain principles of his subject, so that
he whose subject is existing things qua existing must be able to state
the most certain principles of all things. This is the philosopher,
and the most certain principle of all is that regarding which it is
impossible to be mistaken; for such a principle must be both the
best known (for all men may be mistaken about things which they do not
know), and non-hypothetical. For a principle which every one must have
who understands anything that is, is not a hypothesis; and that
which every one must know who knows anything, he must already have
when he comes to a special study. Evidently then such a principle is
the most certain of all; which principle this is, let us proceed to
say. It is, that the same attribute cannot at the same time belong and
not belong to the same subject and in the same respect; we must
presuppose, to guard against dialectical objections, any further
qualifications which might be added. This, then, is the most certain
of all principles, since it answers to the definition given above. For
it is impossible for any one to believe the same thing to be and not
to be, as some think Heraclitus says. For what a man says, he does not
necessarily believe; and if it is impossible that contrary
attributes should belong at the same time to the same subject (the
usual qualifications must be presupposed in this premiss too), and
if an opinion which contradicts another is contrary to it, obviously
it is impossible for the same man at the same time to believe the same
thing to be and not to be; for if a man were mistaken on this point he
would have contrary opinions at the same time.
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