Works of St. Anselm, tr. by Sidney Norton Deane, , at sacred-texts.com
There is a certain Nature through which whatever is exists, and which exists through itself, and is the highest of all existing beings.
THEREFORE, not only are all good things such through something that is one and the same, and all great things such through something that is one and the same; but whatever is, apparently exists through something that is one and the same. For, everything that is, exists either through something, or through nothing. But nothing exists through nothing. For it is altogether inconceivable that anything should not exist by virtue of something.
Whatever is, then, does not exist except through something. Since this is true, either there is one being, or there are more than one, through which all things that are exist. But if there are more than one, either these are themselves to be referred to some one being, through which they exist, or they exist separately, each through itself, or they exist mutually through one another.
But, if these beings exist through one being, then all things do not exist through more than one, but rather through that one being through which these exist.
If, however, these exist separately, each through itself, there is, at any rate, some power or property of existing through self (existendi per se), by which p. 41 they are able to exist each through itself. But, there can be no doubt that, in that case, they exist through this very power, which is one, and through which they are able to exist, each through itself. More truly, then, do all things exist through this very being, which is one, than through these, which are more than one, which, without this one, cannot exist.
But that these beings exist mutually through one another, no reason can admit; since it is an irrational conception that anything should exist through a being on which it confers existence. For not even beings of a relative nature exist thus mutually, the one through the other. For, though the terms master and servant are used with mutual reference, and the men thus designated are mentioned as having mutual relations, yet they do not at all exist mutually, the one through the other, since these relations exist through the subjects to which they are referred.
Therefore, since truth altogether excludes the supposition that there are more beings than one, through which all things exist, that being, through which all exist, must be one. Since, then, all things that are exist through this one being, doubtless this one being exists through itself. Whatever things there are else then, exist through something other than themselves, and this alone through itself. But whatever exists through another is less than that, through which all things are, and which alone exists through itself. Therefore, that which exists through itself exists in the greatest degree of all things.
There is, then, some one being which alone exists in the greatest and the highest degree of all. But that which is greatest of all, and through which exists whatever is good or great, and, in short, whatever p. 42 has any existence ‑‑ that must be supremely good, and supremely great, and the highest of all existing beings.