Guide for the Perplexed, by Moses Maimonides, Friedländer tr. [1904], at sacred-texts.com
You are acquainted with the well-known principle of the philosophers that God is the intellectus, the ens intelligens, and the ens intelligibile. These three things are in God one and the same, and do not in any way constitute a plurality. We have also mentioned it in our larger work, "Mishneh Torah," and we have explained there that it is a fundamental principle of our religion, namely, that He is absolutely one, that nothing combines with Him; that is to say, there is no Eternal thing besides Him. On that account we say ḥai adonay, "the Lord liveth" (Ruth iii. 13), and not ḥe adonay, "the life of the Lord," for His life is not a thing distinct from His essence, as we have explained in treating of the inadmissibility of the attributes. There is no doubt that he who has not studied any works on mental philosophy, who has not comprehended the nature of the mind, who has no knowledge of its essence, and considers it in no other way than he would consider the nature of whiteness and of blackness, will find this subject extremely difficult, and to him our principle that the intellectus, the intelligens, and the intelligibile, are in God one and the same thing, will appear as unintelligible as if we said that the whiteness, the whitening substance, and the material which is whitened are one and the same thing. And, indeed, many ignorant people refute at once our principle by using such comparisons. Even amongst those who imagine that they are wise, many find this subject difficult, and are of opinion that it is impossible for the mind to grasp the truth of this proposition, although it is a demonstrated truth, as has been shown by Metaphysicians. I will tell you now what has been proved. Man, before comprehending a thing, comprehends it in potentia (δυνάμει) when, however, he comprehends a thing, e.g., the form of a certain tree which is pointed out to him, when he abstracts its form from its substance, and reproduces the abstract form, an act performed by the intellect, he comprehends in reality (ἐνεργείᾳ), and the intellect which he has acquired in actuality, is the abstract form of the tree in man's mind. For in such a case the intellect is not a thing distinct from the thing comprehended. It is therefore clear to you that the thing comprehended is the abstract form of the tree, and at the same time it is the intellect in action: and that the intellect and the abstract form of the tree are not two different things, for the intellect in action is nothing but the thing comprehended, and that agent by which the form of the tree has been turned into an intellectual and abstract object, namely, that which comprehends, is undoubtedly the intellect in action.
[paragraph continues] All intellect is identical with its action: the intellect in action is not a thing different from its action, for the true nature and assence of the intellect is comprehension, and you must not think that the intellect in action is a thing existing by itself, separate from comprehension, and that comprehension is a different thing connected with it: for the very essence of the intellect is comprehension. In assuming an intellect in action you assume the comprehension of the thing comprehended. This is quite clear to all who have made themselves familiar with the figurative language common to this discipline. You therefore accept it as proved that the intellect consists in its action, which is its true nature and essence. Consequently the very thing by which the form of that tree has been made abstract and intelligible, viz., the intellect, is at the same time the intelligens, for the intellect is itself the agens which abstracts the form and comprehends it, and that is the action, on account of which it is called the intelligens; but itself and its action are identical: and that which is called intellect in action consists [in the abovementioned instance] of nothing else but of the form of the tree. It must now be obvious to you that whenever the intellect is found in action, the intellect and the thing comprehended are one and the same thing; and also that the function of all intellect, namely, the act of comprehending, is its essence. The intellect, that which comprehends and that which is comprehended, are therefore the same, whenever a real comprehension takes place. But, when we speak of the power of comprehension, we necessarily distinguish two things: the power itself, and the thing which can be comprehended: e.g., that hylic intellect of Zaid is the power of comprehension, and this tree is, in like manner, a thing which is capable of being comprehended; these, undoubtedly, are two different things. When, however, the potential is replaced by the actual, and when the form of the tree has really been comprehended, the form comprehended is the intellect, and it is by that same intellect, by the intellect in action, that the tree has been converted into an abstract idea, and has been comprehended. For everything in which a real action takes place exists in reality. On the other hand, the power of comprehension, and the object capable of comprehension are two things; but that which is only potential cannot be imagined otherwise than in connexion with an object possessing that capacity, as, e.g., man, and thus we have three things: the man who possesses the power, and is capable of comprehending; that power itself, namely, the power of comprehension, and the object which presents itself as an object of comprehension, and is capable of being comprehended; to use the foregoing example, the man, the hylic intellect, and the abstract form of the tree, are three different things. They become one and the same thing when the intellect is in action, and you will never find the intellect different from the comprehensible object, unless the power of comprehending and the power of being comprehended be referred to. Now, it has been proved, that God is an intellect which always is in action, and that--as has been stated, and as will be proved hereafter--there is in Him at no time a mere potentiality, that He does not comprehend at one time, and is without comprehension at another time, but He comprehends constantly; consequently, He and the things comprehended are one and the same thing, that is to say, His essence: and the act of comprehending because of which it is said that He comprehends,
is the intellect itself, which is likewise His essence, God is therefore always the intellectus, the intelligens, and the intelligibile.
We have thus shown that the identity of the intellect, the intelligens and the intelligibile, is not only a fact as regards the Creator, but as regards all intellect, when in action. There is, however, this difference, that from time to time our intellect passes over from mere potentiality to reality, and that the pure intellect, i.e., the active intellect, finds sometimes obstacles, though not in itself, but accidentally in some external cause. It is not our present intention to explain this subject, but we will merely show that God alone, and none besides Him, is an intellect constantly in action, and there is, neither in Himself nor in anything beside Him, any obstacle whereby His comprehension would be hindered. Therefore He always includes the intelligens, the intellectus, and the intelligibile, and His essence is at the same time the intelligens, the intelligibile, and the intellectus, as is necessarily the case with all intellect in action.
We have reiterated this idea in the present chapter because it is exceedingly abstruse, and I do not apprehend that the reader will confound intellectual comprehension with the representative faculty--with the reproduction of the material image in our imagination, since this work is designed only for those who have studied philosophy, and who know what has already been said on the soul and its faculties.