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CONCERNING BEING AND ESSENCE 31 as he says in his Metaphysics?* And such difference is called simple difference, because it is taken from what is a part of the quiddity of the thing, namely, from the form. But since immaterial substances are simple quiddities, difference in them cannot be taken from that which is a part of the quiddity but from the whole quiddity. And therefore in the beginning of the De Anima 37 Avicenna says that "simple difference . . . is not possessed except in those species the essences of which are composed of matter and form." Similarly also in these sub- stances genus is taken from the whole essence, yet in a dif- ferent mode. For one separate substance agrees with others in immateriality, and these substances differ from one another in their grade of perfection according to their recession from potentiality and their accession to pure act. And therefore in them genus is appropriated from that which ensues from them inasmuch as they are immaterial, as intellectuality or something of this sort. Difference, however, is appropriated from that which ensues from the grade of perfection in them and this is unknown to us. And yet it is not necessary that these differ- ences be accidental, because they are according to greater and less perfection which does not diversify species; for the grade of perfection in receiving the same form does not diversify species, just as more white and less white in participating in the same principle of whiteness (does not), but a different grade of perfection in the very forms or natures participated does diversify species, as for instance nature proceeds by grades from plants to animals through certain (levels) which are mediate between animals and plants, according to the Philosopher in the eighth book of the De Animalibus?* Nor again is it neces- sary that the division of intellectual substances be always 88 Avicenna, Met., V, 5, f. 90 b A. 87 Avicenna, DC An., I, i, f. i b E. 88 Aristotle, De Hist. Animal., VIII, 588 b 4-14.