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CONCERNING BEING AND ESSENCE 5 it follows that essence signifies something common to all na- tures by which diverse beings are disposed in different genera and species, as for instance humanity is the essence of man and so for others. And because that by means of which the thing is constituted in its proper genus or species is that which is signified by the definition indicating what the thing is, hence it is that the name essence has been changed by philosophers into the name quiddity. And this is what the Philosopher fre- quently calls "quod quid erat esse" 5 that is, that by virtue of which a thing (anything) has to be what it is (something), (id li fjv elvai). And indeed it is called form according as by means of form the certitude of any single thing is signified, as Avicenna remarks in the second part of his Metaphysics* This is called by another name, nature, accepting nature ac- cording to the first of the four modes assigned by Boethius in his book DC Duabus Naturis? namely, according as nature is said to be all that which can be comprehended by the intellect in any mode whatsoever; for a thing is not intelligible except by virtue of its definition and essence. And thus also the Philosopher in the Fourth book of his Metaphysics* says that every substance is a nature. But the name nature taken in this sense is seen to signify the essence of a thing inasmuch as it has a disposition (ordinem) towards an operation proper to the thing, since no thing is lacking in its proper operation. Indeed the name quiddity is taken from that which signifies the definition; but it is called essence according as by virtue of it and in it being has existence (esse). But because being is asserted absolutely and primarily of 5 Aristotle, An. Post. I, 22, 82 b 38; De An. Ill, 6, 430 b 28; Met, Z, 1028 b 34. 6 Avicenna, perhaps Met. Ill, 5, f. 80 b; certitudo: essence. 7 Boethius, De Persona et Duabus Nat, c. I, PL, t. 64, col. 134160. 8 Aristotle, Metaphysics, A> 1014 b 35.