Where, then, is virtue, if nothing depends on
ourselves? But it is above all things absurd that vices should be in the
power of the agents, and that no one should do wrong except by deliberate
consent to do so, and yet that this should not be the case with virtue;
all the consistency and firmness of which depends on the things to which
it has assented, and which it has approved. And altogether it is necessary
that something should be perceived before we act, and before we assent to
what is perceived; wherefore, he who denies the existence of perception or
assent, puts an end to all action in life.

XIII. Now let us examine the arguments which are commonly advanced by this
school in opposition to these principles. But, first of all, you have it
in your power to become acquainted with what I may call the foundations of
their system. They then, first of all, compound a sort of art of those
things which we call perceptions, and define their power and kinds; and at
the same time they explain what the character of that thing which can be
perceived and comprehended is, in the very same words as the Stoics. In
the next place, they explain those two principles, which contain, as it
were, the whole of this question; and which appear in such a manner that
even others may appear in the same, nor is there any difference between
them, so that it is impossible that some of them should be perceived, and
that others should not be perceived; but that it makes no difference, not
only if they are in every part of the same character, but even if they
cannot be distinguished.

And when these principles are laid down, then these men comprehend the
whole cause in the conclusion of one argument. But this conclusion, thus
compounded, runs in this way: "Of the things which are seen, some are true
and some are false; and what is false cannot be perceived, but that which
appears to be true is all of such a character that a thing of the same
sort may seem to be also false. And as to those things which are perceived
being of such a sort that there is no difference between them, it cannot
possibly happen that some of them can be perceived, and that others
cannot; there is, then, nothing seen which can really be perceived."

But of the axioms which they assume, in order to draw the conclusions
which they desire, they think that two ought to be granted to them; for no
one objects to them. They are these: "That those perceptions which are
false, cannot