In God there is perfect knowledge. The more immaterial a being is, the more intelligent it is. Unintelligent nature is limited by and immersed in matter; but intelligent natures have a more extended sway, and are less contracted by matter; and hence in proportion as forms are immaterial they approach, so to speak, to a kind of infinitude, and to a share of intellectual perfection. God, Who is absolutely immaterial, is intelligent to an infinitely perfect degree.
God knows Himself by Himself, because, being Pure Actuality, and having nothing potential, the power of knowing and the object known are in all respects the same in Him; nor is He ever without intelligible species, as we are, when our mind is in a state of potentiality as regards understanding, for the Intelligible Species of the Divine Intellect are the Divine Intellect itself, and no other. Thus God understands Himself perfectly, for as actuality and intelligibility correspond to each other, and God is the Pure Act, free from matter and all that can be called potential, consequently He is intelligent perfectly, and knows Himself absolutely.
The Intelligence of God is His Substance. Were it otherwise the Divine Substance would have a relation of potentiality to something else, for to understand is the perfection of the act of an intelligent being, and this in God cannot be; and since in God there is no form which can be distinct from His Existence, it follows that His Intelligence is His Essence and His Existence: therefore in God the Intelligent Mind, that which is understood, the intelligible species, and He Who understands, are altogether One and the Same; nor when we say the Intelligent God do we mean to place any multiplicity in His Substance.
Things other than Himself God knows, because He perfectly knows Himself, and all those things to which His Power extends, for since He is the First Cause of all things, the effects pre-existing in the Cause are in His Intelligence. He knows Himself in Himself, and others He knows not in others but in His own Substance; for His Essence is a Species measured by God Himself, and not by others; for He comprehends the species of all things. God’s knowledge of Himself extends to all other things, because He contains all perfections, and it is evident that knowledge of what is imperfect is contained in the knowledge of what is perfect, as, for instance, knowledge of man contains that of animal. Since, then, every nature consists in some [created] participation of the Divine Perfections, God would not perfectly know Himself unless He knew every modification of existence, and how His perfections are shared by others, as contrarily has been shown above. Nor is His knowledge in any way discursive or successive, for God sees everything in One, that is, in Himself, and He sees effects in Himself as their Cause, and hence His knowledge is not discursive. His knowledge is the Cause of all things, as the knowledge of the artificer is the cause of his work, joined to his will to carry out a determined effect. This knowledge in God as the Cause is called the Knowledge of Approbation.
Moreover, God knows things that exist not, because whatever He or the creature might do, even of only possible things, yet, as such, so far He knows them. Things not actually in existence, but which have been or will be, God knows with the Knowledge of Vision, for all are before Him, and with a single glance He comprehends all time; while things which are only possible, and neither are, nor were, nor will be, He knows by Knowledge of Simple Intelligence.
God knows evil also, for He Who knows anything perfectly must know also what may befall it. Unless God knew the evil that might befall any good thing, the good could not be perfectly known to Him; and because evil is the privation of good, by the very fact that God knows good He knows also evil, as by light is known darkness.
And God likewise knows single things, for all the perfections of creatures exist in Him in a more eminent mode, and His knowledge, which is the Cause of all things, must extend as far as His Causality, And because the power of God extends not only to forms but to matter also, His knowledge must reach to the least details which receive their individuation from matter; thus Cause may be able to contain the knowledge of all its effects, in general and in particular.
God knows infinity with the Knowledge of Vision, for He knows all that is within His power, and in that of creatures; the thoughts and affections infinitely multiplied in rational creatures, and which last for ever, He knows all. This knowledge extends in relation to form, which is the principle of intelligence. Since, therefore, the Divine Essence, by which the Divine Intellect understands, is a Species adequate to comprehend all actual or possible beings, it follows that the knowledge of God extends infinitely, not only to things in general, but to each individual detail of everything, and that it extends infinitely to all distinctions and differences.
As God knows all, He sees even future contingent things, not only in their causes, but in their actuality and determination, which to us is only conjectural knowledge. God sees all at once by His Eternal Intuition, and not successively, and hence all things are certain in His Mind, contingencies being such in relation to their immediate causes.
God likewise knows propositions, not by composition and division, as is the way with us, but by His Essence He knows material things immaterially and compound things simply, which is not the case in our human mind, for we pass in a discursive manner from one subject to another, and in our mind one intelligible species does not represent anything but its own object. Since the Substance of God is immutable, His knowledge cannot change, for whatever is, or can be, He knows from eternity.
God has of Himself a speculative knowledge because He is not operable, but of created things, He has speculative and practical knowledge; the first as He knows all things speculatively, the second as regards things that belong to time. Evil also, though not operable to Him, falls under His knowledge, inasmuch as He permits, or prevents, or punishes it, just as we may say that disease comes under the practical knowledge of the physician to remove and cure.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni