P43: He who has a true idea at the same time knows that he has a true idea, and cannot doubt the truth of the thing.

Dem.: An idea true in us is that which is adequate in God/Nature insofar as he is explained through the nature of the human Mind (by P11 C). Let us posit, therefore, that there is in God/Nature, insofar as he is explained through the nature of the human Mind, an adequate idea, A. Of this idea there must necessarily also be in God/Nature an idea which is related to God/Nature in the same way as idea A (by P20, whose demonstration is universal {NS: and can be applied to all ideas}).

But idea A is supposed to be related to God/Nature insofar as he is explained through the nature of the human Mind; therefore the idea of idea A must also be related to God/Nature in the same way, i.e. (by the same P11 C), this adequate idea of idea A will be in the Mind itself which has the adequate idea A. And so he who has an adequate idea, or (by P34) who knows a thing truly, must at the same time have an adequate idea, or true knowledge, of his own knowledge. I.e. (as is manifest through itself), he must at the same time be certain, q.e.d.

Schol.: In P21 S I have explained what an idea of an idea is. But it should be noted that the preceding proposition is sufficiently manifest through itself. For no one who has a true idea is unaware that a true idea involves the highest certainty. For to have a true idea means nothing other than knowing a thing perfectly, or in the best way. And of course no one can doubt this unless he thinks that an idea is something mute, like a picture on a tablet, and not a mode of thinking, viz. the very {act of} understanding. And I ask, who can know that he understands some thing unless he first understands it? I.e., who can know that he is certain about some thing unless he is first certain about it? What can there be which is clearer and more certain than a true idea, to serve as a standard of truth? As the light makes both itself and the darkness plain, so truth is the standard both of itself and of the false.

By this I think we have replied to these questions: if a true idea is distinguished from a false one, {NS: not insofar as it is said to be a mode of thinking, but} only insofar as it is said to agree with its object, then a true idea has no more reality or perfection than a false one (since they are distinguished only through the extrinsic denomination {NS: and not through the intrinsic denomination})—and so, does the man who has true ideas {NS: have any more reality or perfection} than him who has only false ideas? Again, why do men have false ideas? And finally, how can someone know certainly that he has ideas which agree with their objects?

To these questions, I say, I think I have already replied. For as far as the difference between a true and a false idea is concerned, it is established from P35 that the true is related to the false as being is to nonbeing. And the causes of falsity I have shown most clearly from P19 to P35 S. From this it is also clear what is the difference between the man who has true ideas and the man who has only false ideas. Finally, as to the last, viz. how a man can know that he has an idea that agrees with its object? I have just shown, more than sufficiently, that this arises solely from his having an idea that does agree with its object—or that truth is its own standard. Add to this that our Mind, insofar as it perceives things truly, is part of the infinite intellect of God/Nature (by P11 C); hence, it is as necessary that the mind’s clear and distinct ideas are true as that God/Nature’s ideas are.