There is thus a contrast in thought between two elements, the universal and the particular, and both of these are essential, I conceive, to the process of thinking. The particular element is usually a sensation, or sensible thing. The universal element is called by Plato an idea.
24. We may perhaps get still further light on the nature of ideas if we view the matter in this way. Every object that we behold is an instance, that is, it is looked upon as not the only case of the kind; other instances are either actual or possible. But all instances must be instances of something. What is that something? That something is an idea. We require a different term from the word instance to mark that of which the instance is, and for this purpose we employ the term idea. The particular thing before us (suppose it is a tree) is an instance; an instance of what? It is an instance of a tree; but is the tree before us of which this is an instance? Certainly it is not. The particular tree is before us; but that of which it is an instance is not before us, not before us as a particular, is not visible to our sense of sight, although present to the mind as an idea or universal. We thus make a distinction between an instance and that of which it is an instance. In fact, here again we find the two elements which are essential to all thought, the particular and the universal. The terms by which we have just designated them are, the instance, and that of which the
