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34 CONCERNING BEING AND ESSENCE a certain essence ensues from their conjunction. Whence the form, although considered in itself (in se) it does not possess the complete ratio of essence, is nevertheless part of a complete essence. But that to which the accident advenes is a being complete in itself (in se), subsisting in its own existence; which certain existence naturally precedes the accident which supervenes to it. And therefore the supervening accident, from its conjunction with that to which it advenes, does not cause that existence in which the thing subsists, through which the thing is a being by virtue of itself (ens per se), but it causes a certain secondary existence, without which the subsisting thing can be conceived, as the first can be conceived without the second. Whence from an accident and a subject is not ef- fected a unity by virtue of itself (unum per se: substantial unity), but a unity by virtue of accident (unum per accident: accidental unity). And therefore from their conjunction a cer- tain essence does not result, as from the conjunction of form with matter. For which reason an accident has neither the ratio of complete essence, nor is it a part of complete essence; but just as it is being in a certain respect (secundum quid), so also it has essence in a certain respect. But because that which is in the greatest degree and most truly asserted in any genus whatsoever is the cause of those things which are posterior in that genus, as for instance fire which is the extreme of hotness is the cause of heat in hot things, as is also said in the second book of the Metaphysics, 40 therefore substance, which is first in the genus of being, hav- ing essence most truly and in the greatest degree, is necessarily the cause of accidents which participate the principle of being only secondarily and, as it were, in a certain respect (secundum quid). This however happens in diverse ways. For since the parts of a substance are matter and form, therefore 40 Aristotle, Met. a, 993 b 24.