Works of St. Anselm, tr. by Sidney Norton Deane, , at sacred-texts.com
How this Being is said to be substance: it transcends all substance and is individually whatever it is.
BUT, if what we have ascertained concerning the simplicity of this Nature is established, how is it substance? For, though every substance is susceptible of admixture of difference, or, at any rate, susceptible of mutation by accidents, the immutable purity of this Being is inaccessible to admixture or mutation, in any form.p. 85
How, then, shall it be maintained that it is a substance of any kind, except as it is called substance for being, and so transcends, as it is above, every substance? For, as great as is the difference between that Being, which is through itself whatever it is, and which creates every other being from nothing, and a being, which is made whatever it is through another, from nothing; so much does the supreme Substance differ from these beings, which are not what it is. And, since it alone, of all natures, derives from itself, without the help of another nature, whatever existence it has, is it not whatever it is individually and apart from association with its creatures?
Hence, if it ever shares any name with other beings, doubtless a very different signification of that name is to be understood in its case.