(1748). The rise of temperature under the influence of enthusiasm and the mental agitation produced by fevers can only be explained on the theory that what we call the soul consists of pure matter. Sensation is an attribute of matter, just like extension and motion. The real nature of matter however transcends the power of our understanding.—Besides these materialistic theories, La Mettrie's works (Systeme d'Epicure; L'homme planta) contain interesting anticipations and suggestions of a theory of evolution. The various forms of life evolve from eternal organic germs under the influence of environment. Desire and need are the forms of energy which make for progress, and beings without needs lack the attribute of mind. Man is the highest being, because he is conscious of the greatest amount of needs.
Von Holbach (1723-1789), a German baron living in Paris, published a purely dogmatic and systematic elaboration of materialism. In his Systbme de la nature(1770) he contends that materialism is the only consistent explanation of the facts of natural science. If motion is a primary property of matter (as Toland had affirmed), and if material phenomena are only explainable by reference to material causes, it follows that it is unnecessary to assume either one or many minds distinct from matter. An appeal to mind is only a sign of ignorance. Thought or consciousness is simply the agitation of the particles of matter, a motion which is similar to fermentation, which is the common basis of all nourishment and growth, motions which are indeed imperceptible, but which are inferred from what is evident to the senses. There is but one science, physics, i. e. the theory of motion. The assumption of two kinds of nature, spiritual and material, is not only unnecessary, but positively harmful. It is
