Every one will admit that we can think of a thought. We remember ourselves remembering, or imagine ourselves remembering, an object or an event, just as we remember or imagine ourselves seeing one. But in the case of a simple idea of sensation, _i.e._ the idea or remembrance of a single undivided sensation, there seems nothing to distinguish the idea of the idea, from the idea of the sensation itself. When I imagine myself thinking of the colour of snow, I am not aware of any difference, even in degree of intensity, between the image then present to my mind of the white colour, and the image present when I imagine myself to be seeing the colour. The case, however, is somewhat different with those combinations of simple ideas which have never been presented to my mind otherwise than as ideas. I have an idea of Pericles; but it is derived only from the testimony of history: the real Pericles never was present to my senses. I have an idea of Hamlet, and of Falstaff; combinations which, though made up of ideas of sensation, never existed at all in the world of sense; they never were anything more than ideas in any mind. Yet, having had these combinations of ideas presented to me through the words of Shakespeare, I have formed what is properly an idea not of an outward object, but of an idea in Shakespeare's mind; and I may communicate my idea to others, whose idea will then be an idea of an idea in my mind. My idea of Pericles, or my idea of any person now alive whom I have never seen, differs from these in the circumstance that I {69} am persuaded that a real object corresponding to the idea does now, or did once, exist in the world of sensation: but as I did not derive my idea from the object, but from some other person's words, my idea is not a copy of the original, but a copy (more or less imperfect) of some other person's copy: it is an idea of an idea.