Aphorism Third, Aphorism Fourth.

Since our perceptions are all of them always such as we feel them, when we perceive one idea in another, it is actually and really there, from the very circumstance of our perceiving it there: hence no one of our judgments taken separately and detached, is false. It has always and necessarily the certitude which belongs inevitably to each of our actual perceptions.

When we see in an idea, or a perception, an element incompatible with those which it included before, this idea is different from what it was, for, such as it was, it excluded this new element which we see there; and, such as it is, it excludes those which are incompatible with it.

That it may then be the same idea which it was before, we must exclude from it the elemen which we see there at present, or if those which are repugnant to it, are misplaced in this idea, they must themselves be excluded from it; that is to say, it must be rendered such as it was, when they were erroneously admitted into it, which is to restore it again to the same state in which it was, before it was changed by a false judgment, without our perceiving it.

When we form a judgment of an idea, when we see in it a new element, one of these four things must necessarily happen: Either the judgment which we now form is consequent to a just idea, in which case it is just; and the idea without changing its nature has only developed and extended itself.

Or it is inconsequent to a just idea, in which case it is false; and the idea is changed, and is become false.

Or it is consequent to an idea already false, then it is false, but the idea is not changed; it is when it has become false previously, that it has changed in relation to what it was primitively.

Or it is inconsequent to a false idea, then it may be just or false; but never certain, for the idea is changed. But it may have become just, such as it was originally, or false, in a manner different from the preceding.