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CONCERNING BEING AND ESSENCE 13 without the determination of a special form, the nature of a thing, by that which is material in respect to its ultimate per- fection. The concept, however, of the difference rational con- sists in the determination of a special form. And from the two concepts (animal and rational) is constituted the concept of the species or definition. And therefore just as a thing constituted from other things does not take the predication of those things, thus neither does the concept take the predication of those con- cepts from which it is constituted, for we do not say that the definition is genus or difference. But, although genus signifies the whole essence of species, yet it does not follow that there is one essence of different species which have the same genus, because the unity of the genus proceeds from its very indetermination and indifference; not, however, because that which is signified by genus is one nature by number in different species to which supervenes something else which is the difference determining it, as for instance form determines matter which is numerically one; but because genus signifies some form, though not determi- nately this or that (form) which difference expresses deter- minately, which is none other than that (form) which is signified indeterminately through genus. And therefore the Commentator says in the twelfth book of the Metaphysics that prime matter is called one through the remotion of all forms (scil. pure potentiality in the order of substance), but genus is called one through the community of its signified form. Whence it is clear that by means of the addition of difference, which removes the indetermination which was the cause of the unity of genus, species remain different by virtue of essence. And because, as has been said, the nature of species is in- determinate in respect to the individual, just as the nature of 16 Averroes, In Met, XII, c. 14, f. 141 a 53 b 18.