Phil. How then is it possible that things perpetually fleeting and variable as our ideas should be copies or images of anything fixed and constant? Or, in other words, since all sensible qualities, as size, figure, colour, &c., that is, our ideas, are continually changing, upon every alteration in the distance, medium, or instruments of sensation; how can any determinate material objects be properly represented or painted forth by several distinct things, each of which is so different from and unlike the rest? Or, if you say it resembles some one only of our ideas, how shall we be able to distinguish the true copy from all the false ones?
Hyl. I profess, Philonous, I am at a loss. I know not what to say to this.
Phil. But neither is this all. Which are material objects in themselves—perceptible or imperceptible?
Hyl. Properly and immediately nothing can be perceived but ideas. All material things, therefore, are in themselves insensible, and to be perceived only by our ideas.
Phil. Ideas then are sensible, and their archetypes or originals insensible?
Hyl. Right.
Phil. But how can that which is sensible be like that which is insensible? Can a real thing, in itself invisible, be like a colour; or a real thing, which is not audible, be like a sound? In a word, can anything be like a sensation or idea, but another sensation or idea?
Hyl. I must own, I think not.
Phil. Is it possible there should be any doubt on the point? Do you not perfectly know your own ideas?
Hyl. I know them perfectly; since what I do not perceive or know can be no part of my idea[818].