3. But what shall be the criterion of this agreement?
It is evident the mind knows not things immediately, but only
by the intervention of the ideas it has of them. Our knowledge, therefore is real only so far as there is a conformity
between our ideas and the reality of things. But what shall be here the criterion? How shall the mind, when it
perceives nothing but its own ideas, know that they agree with things themselves? This, though it seems not to
want difficulty, yet, I think, there be two sorts of ideas that we may be assured agree with things.