constitute the _efficient_ cause of the present oak. (4) And there must be a final stage to which the whole process of growth is relative, in which the germ or sapling is no longer becoming but is an adult oak bearing fresh acorns. This is the _end_ of the process. One would not be going far wrong in saying that Aristotle’s biological cast of thought leads him to conceive of this "end" in the case of reproduction as a sub-conscious purpose, just as the workman’s thought of the result to be attained by his action forms a conscious directing purpose in the case of manufacture. Both in Nature and in "art" the "form," the "efficient cause," and the "end" tend to coalesce. Thus in Nature "a man begets a man," organic beings give birth to other organic beings of the same kind, or, in the technical language of the Aristotelian theory of Causation, the efficient cause produces, as the "end" of its action, a second being having the same "form" as itself, though realised in different "matter," and numerically distinct from itself. Thus the efficient cause (_i.e._ the parent) is a "form" realised in matter, and the "end" is the same "form" realised in other matter. So in "products of art" the true "source of the process" is the "form" the realisation of which is the "end" or final cause, only with this difference, that as efficient cause the "form" exists not in the material but by way of "idea" or "representation" in the mind of the craftsman. A house does not produce another house, but the house as existing in "idea" in the builder’s mind sets him at work building, and so produces a corresponding house in brick or stone. Thus the ultimate opposition is between the "cause as matter," a passive and inert substratum of change and development and the "formal" cause which, in the sense just explained, is one with both the "efficient" or starting-point, and the "end" or goal of development. It will, of course, be seen that individual bearers of "forms" are indispensable in the theory; hence the notion of _activity_ is essential to the causal relation. It is a relation between things, not between events. Aristotle has no sense of the word cause corresponding to Mill’s conception of a cause as an event which is the uniform precursor of another event.