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CONCERNING BEING AND ESSENCE 23 substance differ in that the essence of a composite substance is not form alone but embraces form and matter, whereas the essence of a simple substance is form alone. And from this two other differences are derived. One is that the essence of a com- posite substance can be signified as a whole or as a part, which happens according to the designation of the matter, as has been stated. And therefore the essence of a composite thing is not predicated in any mode whatsoever of the composite thing itself; for it cannot be said that man is his quiddity. But the essence of a simple thing, which is its form, cannot be signified except as a whole, since there is nothing there except the form as form receiving, and, therefore, in whatever mode the essence of a simple substance is taken it is predicated of the substance. Whence Avicenna says 24 that the quiddity of a simple (sub- stance) is itself simple, because there is not anything else receptive of the quiddity. The second difference is that the es- sences of composite things, seeing that they are received in designated matter, are multiplied according to its division, whence it results that some things are the same in species and diverse numerically. But since the essence of simple substance is not received in matter, there cannot be there any such mul- tiplication. And therefore it follows necessarily that in these substances more than one individual of the same species are not found, but however many individuals there are, just so many are the species, as Avicenna expressly says. 25 (scil. "A species of this mode is one in number.") And indeed substances of this sort, although they are forms alone without matter, still do not have an entire simplicity of nature so that they are pure act; on the contrary, they have a mixture of potency, which is evident thus: for whatsoever does not belong to the concept of essence or quiddity is some- 24 Avicenna, Met., V, 5, f. 90 a F. 25 Avicenna, Met., V, 2, f. 87 a A.