VIDENTLY 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.