Selection from St. Thomas Aquinas, On Being and Essence, trans. Armand Maurer, C.S.B., cc. 77-80
77. Whatever is not of the understood content of an essence or quality is something which comes from without and makes a composition with the essence, because no essence can be understood without the things which are parts of it. Now, every essence or quiddity can be understood without anything being understood about its existence. For I can understand what a man is, or what a phoenix is, and yet not know whether they have existence in the real world. It is clear, therefore, that existence is other than essence or quiddity, unless perhaps there exists a thing whose quiddity is its existence.
78. And there can be but one such thing, the First Thing, because it is impossible to plurify a thing except: (1) by the addition of some difference, as the nature of the genus is multiplied in its species, or (2) by the reception of a form into diverse matters, as the nature of the species is multiplied in diverse individuals, or (3) by this: that one is absolute and the other is received into something; for example, if there were a separated heat, it would by virtue of its very separation be other than heat which is not separated. Now, if we posit a thing which is existence alone, such that this existence is subsistent, this existence will not receive the addition of a difference because it would no longer be existence alone, but existence plus some form. And much less will it receive the addition of matter because it would no longer be a subsistent existence, but a material existence. Whence it remains that such a thing, which is its own existence, cannot be but one.
79. Whence it is necessary, that in every thing other than this one its existence be other than its quiddity, or its nature, or its form. Whence it is necessary that existence in the intelligences be something besides the form, and this is why it was said that an intelligence is form and existence.
80. Now, whatever belongs to a thing is either caused by the principles of its nature, as the ability to laugh in man, or comes to it from some extrinsic principle, as light in the air from the influence of the sun. But it cannot be that the existence of a thing is caused by the form or quiddity of that thing ─ I say caused as by an efficient cause ─ because then something would be its own cause, and would bring itself into existence, which is impossible. It is therefore necessary that every such thing, the existence of which is other than its nature, have its existence from some other thing. And because every thing which exists by virtue of another is led back, as to its first cause, to that which exists by virtue of itself, it is necessary that there be some thing which is the cause of the existence of all things because it is existence alone. Otherwise, there would be an infinite regress among causes, since every thing which is not existence alone has a cause of its existence, as has been said. It is clear, therefore, that an intelligence is form and existence, and that it has existence from the First Being, which is existence alone. And this is the First Cause, which is God.