This is the conception of "vital force" which comes straight from the philosophy of Aristotle. It is a fundamental proposition of that philosophy that a natural object is composed of two constituents--the one its matter, conceived as inert or even, to a certain extent, opposed to orderly and purposive motion; the other its form, conceived as a quasi-spiritual something, containing or conditioning the actual activities of the body and the potentiality of its possible activities. I am disposed to think that the prominence of this conception in Aristotle's theory of things arose from the circumstance that he was to begin with and throughout his life, devoted to biological studies. In fact it is a notion which must force itself upon the mind of any one who studies biological phenomena, without reference to general physics, as they now stand. Everybody who observes the obvious phenomena of the development of a seed into a tree, or of an egg into an animal, will note that a relatively formless mass of matter gradually grows, takes a definite shape and structure, and, finally, begins to perform actions which contribute towards a certain end, namely, the maintenance of the individual in the first place, and of the species in the second. Starting from the axiom that every event has a cause, we have here the _causa finalis_ manifested in the last set of phenomena, the _causa materialis_ and _formalis_ in the first, while the existence of a _causa efficiens_ within the seed or egg and its product, is a corollary from the phenomena of growth and metamorphosis, which proceed in unbroken succession and make up the life of the animal or plant. Thus, at starting, the egg or seed is matter having a "form" like all other material bodies. But this form has the peculiarity, in contradistinction to lower substantial "forms," that it is a power which constantly works towards an end by means of living organisation.