In all sciences and in all arts these sources are but three--observation, consciousness, and hypothesis. The physical sciences, being only secondarily conversant with mind, draw their premises almost exclusively from observation or hypothesis. Those which treat only of magnitude and number, or, as they are usually called, the pure sciences, draw them altogether from hypothesis. The mathematician does not measure the radii of a circle in order to ascertain that they are all equal: he infers their equality from the definition with which he sets out. Those which abstain from hypothesis depend on observation. It is by observation that the astronomer ascertains the motions of the planets, the botanist classifies plants, and the chemist discovers the affinities of different bodies. They disregard almost entirely the phenomena of consciousness. The physical _arts_ are almost exclusively based on observation. As their object is to produce positive effects, they trust as little as possible to hypothesis; and the mental phenomena which they have to consider are generally few and simple. The art of navigation, the art of mining, or the art of fortification, might be taught by a man who had never studied seriously the operations of his own mind. On the other hand, the mental sciences and the mental arts draw their premises principally from consciousness. The subjects with which they are chiefly conversant are the workings of the human mind. And the only mind whose workings a man really knows is his own. When he wishes to ascertain the thoughts and the feelings of others, his first impulse always is, to endeavour to suppose himself in what he believes to be their situation, and to consider how he himself would then think and feel. His next impulse is to infer that similar moral and intellectual processes are taking place in them. If he be a cautious observer, he endeavours to correct this inference by examining their countenances, their words, and their actions. But these are uncertain symptoms, often occasioned by a state of mind different from that which they appear to indicate; and often employed for the purpose of concealment or of deception.