Thus, although the man be assumed to acquiesce in what is false, we shall never say that he is certain.

For by certainty we mean something positive (II.xliii.and note), not merely the absence of doubt.
However, in order that the foregoing proposition may be fully explained, I will draw attention to a few additional points, and I will furthermore answer the objections which may be advanced against our doctrine.

Lastly, in order to remove every scruple, I have thought it worth while to point out some of the advantages, which follow therefrom.

I say "some," for they will be better appreciated from what we shall set forth in the fifth part.
I begin, then, with the first point, and warn my readers to make an accurate distinction between an idea, or conception of the mind, and the images of things which we imagine.

It is further necessary that they should distinguish between idea and words, whereby we signify things.