He who has a true idea knows at the same time that he has a true idea, nor can he doubt the truth of the thing. For no one who has a true idea is ignorant that a true idea involves the highest certitude; to have a true idea signifying just this, to know a thing perfectly or as well as possible. No one, in fact, can doubt this, unless he supposes an idea to be something dumb, like a picture on a tablet, instead of being a mode of thought, that is to say, intelligence itself. Moreover, I ask who can know that he understands a thing unless he first of all understands that thing? that is to say, who can know that he is certain of anything unless he is first of all certain of that thing? Then, again, what can be clearer or more certain than a true idea as the standard of truth? Just as light reveals both itself and the darkness, so truth is the standard of itself and of the false. _Of Falsity_ There is nothing positive in ideas which can constitute a form of falsity. But falsity cannot consist in absolute privation (for we say that minds and not bodies err and are mistaken); nor can it consist in absolute ignorance, for to be ignorant and to be in error are different. Falsehood, therefore, consists in the privation of knowledge which is involved by inadequate knowledge of things or by inadequate and confused ideas. For instance, men are deceived because they think themselves free, and the sole reason for thinking so is that they are conscious of their own actions, and ignorant of the causes by which those actions are determined. Their idea of liberty therefore is this--that they know no cause for their own actions; for as to saying that their actions depend upon their will, these are words to which no idea is attached. What the will is, and in what manner it moves the body, every one is ignorant, for those who pretend otherwise, and devise seats and dwelling-places of the soul, usually excite our laughter or disgust. Just in the same manner, when we look at the sun, we imagine its distance from us to be about 200 feet; the error not consisting solely in the imagination, but arising from our not knowing what the true distance is when we imagine, and what are the causes of our imagination. For although we may afterwards know that the sun is more than 600 diameters of the earth distant from us, we still imagine it near us, since we imagine it to be so near, not because we are ignorant of its true distance, but because a modification of our body involves the essence of the sun, in so far as our body itself is affected by it. _The Origin and Nature of Confused Ideas_