Resting on these clear perceptions, the natural philosophy of Locke falls into two parts, one strictly physical and scientific, the other critical and psychological. In respect to the composition of matter, Locke accepted the most advanced theory of his day, which happened to be a very old one: the theory of Democritus that the material universe contains nothing but a multitude of solid atoms coursing through infinite space: but Locke added a religious note to this materialism by suggesting that infinite space, in its sublimity, must be an attribute of God. He also believed what few materialists would venture to assert, that if we could thoroughly examine the cosmic mechanism we should see the demonstrable necessity of every complication that ensues, even of the existence and character of mind: for it was no harder for God to endow matter with the power of thinking than to endow it with the power of moving. In the atomic theory we have a graphic image asserted to describe accurately, or even exhaustively, the intrinsic constitution of things, or their primary qualities. Perhaps, in so far as physical hypotheses must remain graphic at all, it is an inevitable theory. It was first suggested by the wearing out and dissolution of all material objects, and by the specks of dust floating in a sunbeam; and it is confirmed, on an enlarged scale, by the stellar universe as conceived by modern astronomy. When today we talk of nuclei and electrons, if we imagine them at all, we imagine them as atoms. But it is all a picture, prophesying what we might see through a sufficiently powerful microscope; the important philosophical question is the one raised by the other half of Locke's natural philosophy, by optics and the general criticism of perception. How far, if at all, may we trust the images in our minds to reveal the nature of external things? On this point the doctrine of Locke, through Descartes,[3] was also derived from Democritus. It was that all the sensible qualities of things, except position, shape, solidity, number and motion, were only ideas in us, projected and falsely regarded as lodged in things. In the things, these imputed or secondary qualities were simply powers, inherent in their atomic constitution, and calculated to excite sensations of that character in our bodies. This doctrine is readily established by Locke's plain historical method, when applied to the study of rainbows, mirrors, effects of perspective, dreams, jaundice, madness, and the will to believe: all of which go to convince us that the ideas which we impulsively assume to be qualities of objects are always, in their seat and origin, evolved in our own heads.