THE UNIVERSE A MACHINE.--Once assume that all phenomena can be interpreted in terms of motion, and add the proposition (already enunciated by Galileo) that motion once set going will proceed for ever, unless some impediment from outside intervenes, and the mechanical view of the universe is complete. The universe is a machine, i.e. a thing that works (1) according to mathematical principles, (2) automatically. ELABORATIONS OF THE MECHANICAL THEORY.--The importance of Descartes lies not in his having invented this conception (we have already seen it in the hands of Leonardo da Vinci, Galileo, and others), but in his having elaborated it. This he did in two directions: (1) he attempted to supply a mechanical theory of the evolution of the world-system; _i.e._ to show how the heavenly bodies came into being by natural and mechanical processes; (2) he applied the mechanical theory to organisms; animals and men were complex machines. (Here, as we have seen, the discovery of Harvey was of prime importance.) It is hardly necessary to describe at length Descartes' mechanical theory of the evolution of the world-system, though an interest attaches to it as being the ancestor of the modern "nebular hypothesis." Matter in whirling motion around fixed centres is the original _datum_ from which Descartes evokes the universe. With regard to the mechanical theory of organisms, Descartes developed it at some length in various treatises. All the functions and actions of animals were regarded by him as entirely involuntary and mechanical. "That the lamb flees at the sight of a wolf happens because the rays of light from the body of the wolf strike the eye of the lamb, and set the muscles in motion by means of the 'reflex' currents of the animal spirits." In the case of human beings, owing to the phenomenon of "consciousness," Descartes felt compelled to assume a "soul"--a _thinking_ substance in reciprocal action with the _material_ substance (of the brain). This, too, is an anticipation of the modern theory of "psycho-physical parallelism." CARTESIANISM.--The ideas of Descartes had considerable influence among his contemporaries, and Cartesianism, as it was called, became fashionable in intellectual circles. It developed a tendency towards free enquiry and independent thought; and it was even more significant as an atmosphere than as a system of ideas. Though in this respect too, it was both important and vital; as we have observed, modern mechanical theories find their parent in Descartes. Nor was it only, we may remark, among philosophers and men of science that Cartesian ideas were popular; they were accepted and elaborated by the religious thinkers who hoped to harmonise and humanise theology and science. Pascal, Bossuet and Fenelon, the finest minds in the French Church, were eager Cartesians.[5] This aspect of the matter, i.e. the significance of Cartesianism for religion, we can for the present postpone.