doctrine, with this modification, that whereas Plato (at least as understood by Aristotle) promulgated a doctrine in which ideas were represented as existing by themselves, and apart from things, Aristotle represented them as implicated in things, and as forming their most essential constituent. The idea, for example, considered as the one does not exist together with the many, but it exists in the many. Unity is essential to multiplicity. If we view ideas as laws, we might say that while Plato, at least as interpreted by Aristotle, regarded the laws as subsisting by themselves, and as constituting a world apart, Aristotle regarded them as inseparably united with the things of which they were the laws. (The individual is the essence in the first and proper sense of the word; only in a secondary sense can the genus be called the essence.) The genus has no existence apart from the individuals, yet although the genus or universal has no existence in and for itself, but only an existence in individuals, it is nevertheless the most significant, and in its nature the most knowable, and the proper object of knowledge. There can be no knowledge without it.
7. Aristotle's third and fourth principles are efficient cause and final cause. Every change from the potential to the actual is brought about by a cause, and this cause is distinguished by Aristotle as the moving or efficient cause, τὸ κινῆσαν. It may either operate from within, as in the case of organised ex-
