It is not, therefore, out of our way, in the examination of our knowledge, to inquire into the truth and certainty of universal propositions.
3.

Certainty twofold--of Truth and of Knowledge.
But that we may not be misled in this case by that which is the danger everywhere, I mean by the doubtfulness of terms, it is fit to observe that certainty is twofold: CERTAINTY OF TRUTH and CERTAINTY OF KNOWLEDGE.

Certainty of truth is, when words are so put together in propositions as exactly to express the agreement or disagreement of the ideas they stand for, as really it is.

Certainty of knowledge is to perceive the agreement or disagreement of ideas, as expressed in any proposition.

This we usually call knowing, or being certain of the truth of any proposition.
4.