On the True and the False by Baruch Spinoza

Let us now examine the true and the false, which indicate to us the fourth, and last, consequence of true belief. Now, in order to do this, we shall first state the definitions of Truth and Falsity. Truth is an affirmation (or a denial) made about a certain thing, which agrees with that same thing; and Falsity is an affirmation (or a denial) about a thing, which does not agree with the thing itself. But this being so, it may appear that there is no difference between the false and the true Idea, or, since the [affirmation or] denial of this or that are mere modes of thought, and [the true and the false Idea] differ in no other way except that the one agrees with the thing, and the other does not, that they are therefore, not really, but only logically different; and if this should be so, one may justly ask, what advantage has the one from his Truth, and what harm does the other incur through his falsity? and how shall the one know that his conception or Idea agrees with the thing more than the other does? lastly, whence does it come that the one errs, and the other does not?

To this it may, in the first place, serve as an answer that the clearest things of all make known both themselves and also what is false, in such a manner that it would be a great folly to ask how we are to become aware of them: for, since they are said to be the clearest of all, there can never be any other clearness through which they might be made clear; it follows, therefore, that truth is made clear through truth, that is through itself; and through it also is falsity made clear; but falsity is never revealed and made manifest through itself. So that anyone who is in possession of the truth cannot doubt that he possesses it, while one who is sunk in falsity or in error can well suppose that he has got at the truth; just as someone who is dreaming can well think that he is awake, but one who is actually awake can never think that he is dreaming.