In God there is Procession according to the mode of intelligible emanation, not according to that of infirm corporeal beings; but because God is above all, no similitude can be found even among spiritual substances for the full expression of things Divine. This Procession is not, therefore, according to local movement, nor according to the action of any cause in the exterior effect, but according to intelligible emanation, that, viz. of a Word, which is intelligible to the mind and remains in it.
And this Procession in God is really generation, because inasmuch as it proceeds by the mode of intelligible action it is an operation of life, and united to its source; and being a conception of the intellect it is really a similitude existing in the same nature; for, in God, to be is the same as to understand. Therefore the Procession of the Word in God is called "Generation," and the Word proceeding is called "Son," while in us the word which proceeds by intelligible operation is not of the same nature with him from whom it proceeds; therefore it does not constitute a generation, properly and completely.
Besides this Procession of the Word in God there is also a Procession of Love. For as in us there is, besides the intelligible operation, a procession of love according to the action of the will, by which the loved is in the lover, so it is also in God, and this Procession cannot be called Generation. The procession which takes place according to similitude is really a generation; for whatever generates must generate that which is like to itself; but the procession which is according to will has the nature of an impulse towards something; hence that which, in the Divinity, proceeds by way of Love does not proceed as generated or as Son, but rather as Breath or Spirit: a name whereby we designate the motion and impulse of life.
Nor is there any other Procession in the Divinity besides those of the Word and of Love; for God is purely intellectual, and in such nature no action is possible except to understand and to will.
And since God understands and wills all by one simple act, there cannot be in Him Procession of the Word from the Word, or of Love from Love; but there is One Word only, and One perfect Love; and in this the perfection of His fecundity is made manifest.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni