Thomas Aquinas, Compendium of Theology

Among things that are not distinct in essence, there can be no distinction according to species, time, or nature. Therefore, since the Word is consubstantial with the Father, He cannot differ from the Father in any of these respects.

There can be no difference according to time. The divine Word is present in God for the reason that God understands Himself, thereby conceiving His intelligible Word. Hence, if at any time there were no Word of God, during that period God would not understand Himself. But God always understood Himself during His whole existence, for His understanding is His existence. Therefore His Word, also, existed always. And so in the rule of Catholic faith we say that the Son of God "is born of the Father before all ages."

According to species, too, it is impossible for the Word of God to differ from God, as though He were inferior; for God does not understand Himself to be less than He is. The Word has a perfect likeness to the Father, because that whereof He is the Word is perfectly understood. Therefore the Word of God must be absolutely perfect according to the species of divinity. Some beings, it is true, that proceed from others, are found not to inherit the perfect species of those from whom they proceed. One way in which this can happen is in equivocal generations: the sun does not generate a sun, but an animal of some kind. To exclude imperfection of this sort from divine generation, we proclaim that the Word is born "God of God." The same thing occurs in another way when that which proceeds from another differs from the latter because of a defect in purity; that is, when something is produced from what is simple and pure in itself by being applied to extraneous matter, and so turns out to be inferior to the original species. Thus, from a house that is in the architect's mind, a house is fashioned in various materials; and from light received in the surface of a body, color results; and from fire, by adding other elements, a mixture is produced; and from a beam of light, by interposing an opaque body, shadow is caused. To exclude any imperfection of this kind from divine generation, we add: "Light of Light." In yet a third way, what proceeds from another can fail to equal the latter's species, because of a deficiency in truth. That is, it does not truly receive the nature of its original, but only a certain likeness thereof; for example, an image in a mirror or in a picture or in a statue; also, the likeness of a thing in the intellect or in one of the senses. For the image of a man is not said to be a true man, but is a likeness of a man; and a stone is not in the soul, as the Philosopher notes, but a likeness of the stone is in the soul. To exclude all this from divine generation, we subjoin: "True God of true God."

Lastly it is impossible for the Word to differ from God according to nature, since it is natural for God to understand Himself. Every intellect has some objects which it naturally understands. Thus, our intellect naturally understands first principles. Much more does God, whose intellectual activity is His existence, naturally understand Himself. Therefore His Word proceeds from Him naturally, not in the way that things proceed otherwise than by natural origin, that is, not in the way that artificial objects, which we are said to make, take shape from us. On the other hand, whatever proceeds from us naturally we are said to generate, for example, a son. Accordingly, to preclude the error of thinking that the Word of God proceeds from God, not by way of nature, but by the power of His will, the phrase is added: "Begotten, not made."