"But that we may not be mis-led in this case, by that which is the danger every-where, I mean by the doubtfulness of Terms, 'tis 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."

that's it.