On the other hand, the movements of material things in space are facts of which we are aware, phenomena of which we are aware through our senses; in a word, they are sense-phenomena. We are aware of them as existing simultaneously and in combination, or as succeeding one upon another; and no truth, even of the most mathematical and exact of the sciences, does, or can do, more than express with mathematical exactness the precise conditions under which these sense-phenomena co-exist or follow one another, or the precise conditions without which such co-existence or sequence cannot take place. A mathematical science dealing with material things states only and always that certain sense-phenomena occur invariably and uniformly under certain conditions. The exact sciences move within the limits of the Uniformity of Nature and the law of Universal Causation; and their subject-matter consists of sense-phenomena, _i.e._ of things which, as known to science, are objects of perception to some mind. But sense-phenomena are not the only mental phenomena of which we are aware: there are ideas which we do not see or handle, or smell or taste, but of which we are nevertheless distinctly conscious. Thought has its movement, ideas have their co-existence and sequences, the association of ideas has its laws. There is a uniformity of human nature as well as of external nature; there are conditions under which certain actions are always performed, and without which they would never be done. Whether the body of propositions in which these conditions are formulated be accorded or denied the name of science, matters little. But it is difficult to see what are the so great differences between these phenomena and sense-phenomena that make the latter amenable and the former insusceptible to scientific treatment. Is it that ideas are invisible? So is weight, yet the gravitation formula is scientific. Is it that thought is impalpable? So is colour, so is sound--yet there are optics and acoustics. Be this as it may, what makes things material susceptible to scientific treatment is a quality which is not peculiar to them, but which is shared by them in common with things immaterial: it is that they are objects of which the mind is immediately aware, phenomena present to some consciousness, and that they are phenomena which appear in consciousness as co-existent and successive in certain definite uniform modes which can be detected by thought and formulated in general propositions, or laws. If the theory of evolution comprehends all things, mind and morals as well as matter and motion; if the law of continuity connects all things together, immaterial as well as material, in a process which moves without break or interruption; it is because all things agree in the fact that they are presented (whether in sense or in idea) to the mind, and because they are presented in the continuity of consciousness.