WE have just seen that the antithesis of Theory and Fact, although it involves the antithesis of Thoughts and Things, is not identical with it. There are other modes of expression also, which involve the same Fundamental Antithesis, more or less modified. Of these, the pair of words which in their relations appear to separate the members of the antithesis most distinctly are _Ideas_ and _Sensations_. We see and hear and touch external things, and thus perceive them by our senses; but in perceiving them, we connect the impressions of sense according to relations of space, time, number, likeness, cause, &c. Now some at least of these kinds of connexion, as space, time, number, may be contemplated distinct from the things to which they are applied; and so contemplated, I term them _Ideas_. And {31} the other element, the impressions upon our senses which they connect, are called _Sensations_. I term space, time, cause, &c., _Ideas_, because they are general relations among our sensations, apprehended by an act of the mind, not by the senses simply. These relations involve something beyond what the senses alone could furnish. By the sense of sight we see various shades and colours and shapes before us, but the _outlines_ by which they are separated into distinct objects of definite forms, are the work of the mind itself. And again, when we conceive visible things, not only as surfaces of a certain form, but as _solid bodies_, placed at various distances in space, we again exert an act of the mind upon them. When we see a body move, we see it move in a path or _orbit_, but this orbit is not itself seen; it is constructed by the mind. In like manner when we see the motions of a needle towards a magnet, we do not _see_ the attraction or force which produces the effects; but we infer the force, by having in our minds the Idea of Cause. Such acts of thought, such _Ideas_, enter into our perceptions of external things.