Works of St. Anselm, tr. by Sidney Norton Deane, , at sacred-texts.com
It cannot suffer change by any accidents [Accidents, as Anselm uses the term, are facts external to the essence of a being, which may yet be conceived to produce changes in a mutable being.]
BUT does not this Being, which has been shown to exist as in every way substantially identical with itself, sometimes exist as different from itself, at any rate accidentally? But how is it supremely immutable, if it can, I will not say, be, but, be conceived of, as variable by virtue of accidents? And, on the other hand, does it not partake of accident, since even this very fact that it is greater than all other natures and that it is unlike them seems to be an accident in its case (illi accidere)? But what is the inconsistency between susceptibility to certain facts, called accidents, and natural immutability, if from the undergoing of these accidents the substance undergoes no change?
For, of all the facts, called accidents, some are understood not to be present or absent without some variation in the subject of the accident ‑‑ all colors, for instance ‑‑ while others are known not to effect any change in a thing either by occurring or not occurring ‑‑ certain relations, for instance. For it is certain that I am neither older nor younger than a man who is not yet born, nor equal to him, nor like him. But I shall be able to sustain and to lose all these relations toward him, as soon as he shall have been born, according as he shall grow, or undergo change through divers qualities.
It is made clear, then, that of all those facts, called accidents, a part bring some degree of mutability in their train, while a part do not impair at all the immutabilityp. 84 of that in whose case they occur. Hence, although the supreme Nature in its simplicity has never undergone such accidents as cause mutation, yet it does not disdain occasional expression in terms of those accidents which are in no wise inconsistent with supreme immutability; and yet there is no accident respecting its essence, whence it would be conceived of, as itself variable.
Whence this conclusion, also, may be reached, that it is susceptible of no accident; since, just as those accidents, which effect some change by their occurrence or non‑occurrence, are by virtue of this very effect of theirs regarded as being true accidents, so those facts, which lack a like effect, are found to be improperly called accidents. Therefore, this Essence is always, in every way, substantially identical with itself; and it is never in any way different from itself, even accidentally. But, however it may be as to the proper signification of the term accident, this is undoubtedly true, that of the supremely immutable Nature no statement can be made, whence it shall be conceived of as mutable.