To get a good grip on what ideas are, be warned of an ambiguity. Some people take an idea to be a form or differentia [= ‘aspect or property’] of our thought; so that we have the idea in our mind only when we are thinking of it, and whenever we think of it again, we have different but similar ideas of the same thing. Others, however, seem to take an idea to be the immediate object of a thought, or to be some kind of permanent form, which continues to exist even when we are not contemplating it. I side with the latter group, and here is why. Our soul always possesses the ability to represent to itself any nature or form when the occasion for thinking of it arises. This ability is permanent, even though the individual thoughts in which it is exercised come and go. And I believe that this ability of our soul, when it expresses some nature, form, or essence, is properly called an idea of the thing; and it is in us—always in us—whether or not we are thinking of the thing. For our soul always expresses God and the universe, and all essences as well as all existences. That requires our soul to have ideas of all those things at all times, which it can do only if ideas are abilities rather than individual mental or events or aspects or properties of such events·. This fits in with my principles, for nothing naturally enters our mind from outside; and it is a bad habit of ours to think of our soul as receiving messenger species, or as if it had doors and windows. We have all these forms in our mind and indeed always have had; because the mind always expresses all its future thoughts, and is already thinking confusedly of everything it will ever think clearly. We couldn’t be taught something unless we already had the idea of it in our mind, the idea being like the matter out of which the thought is formed. Plato understood this very well, when he put forward his doctrine of reminiscence. The latter is very sound, provided we take it in the right way—cleansing it of the error about pre-existence, and not imagining that if a soul takes in and thinks about something now it must at some earlier time have clearly known and thought about it. He also confirmed his opinion by a beautiful experiment. He introduces a small boy whom he gradually leads to an acceptance of very difficult geometrical truths about incommensurables, without teaching him anything, only asking him an orderly sequence of suitable questions. This shows that our souls have virtual knowledge of all these things; that to grasp truths they need only to have their attention drawn to them; and thus that our souls at leas thave the ideas on which those truths depend. They might even be said to possess these truths, if we consider the truths as relations between ideas.