A true idea[18] (for we possess a true idea) is something different from its correlate (_ideatum_); thus a circle is different from the idea of a circle. The idea of a circle is not something having a circumference and a center, as a circle has; nor is the idea of a body that body itself. Now, as it is something different from its correlate, it is capable of being understood through itself; in other words, the idea, in so far as its actual essence (_essentia formalis_) is concerned, may be the subject of another subjective essence. And, again, this second subjective essence will, regarded in itself, be something real and capable of being understood; and so on indefinitely. For instance, the man Peter is something real; the true idea of Peter is the reality of Peter represented subjectively, and is in itself something real, and quite distinct from the actual Peter. Now, as this true idea of Peter is in itself something real, and has its own individual existence, it will also be capable of being understood--that is, of being the subject of another idea which will contain by representation all that the idea of Peter contains actually. And, again, this idea of the idea of Peter has its own individuality, which may become the subject of yet another idea; and so on indefinitely. This every one may make trial of for himself, by reflecting that he knows what Peter is, and also knows that he knows, and further knows that he knows that he knows, etc. Hence, it is plain that, in order to understand the actual Peter, it is not necessary first to understand the idea of Peter, and still less the idea of the idea of Peter. This is the same as saying that in order to know, there is no need to know that we know, much less to know that we know that we know. This is no more necessary than to know the nature of a circle before knowing the nature of a triangle. But with these ideas the contrary is the case; for in order to know that I know, I must first know. Hence it is clear that certainty is nothing else than the subjective essence of a thing: in other words, the mode in which we perceive an actual reality is certainty. Further, it is also evident that for the certitude of truth no further sign is necessary beyond the possession of a true idea; for, as I have shown, it is not necessary to know that we know that we know....