Objection 1. It would seem that the higher and lower reason are distinct powers. For Augustine says (De Trin. xii, 4,7), that the image of the Trinity is in the higher part of the reason, and not in the lower. But the parts of the soul are its powers. Therefore the higher and lower reason are two powers.
Objection 2. Further, nothing flows from itself. Now, the lower reason flows from the higher, and is ruled and directed by it. Therefore the higher reason is another power from the lower.
Objection 3. Further, the Philosopher says (Ethic. vi, 1) that "the scientific part" of the soul, by which the soul knows necessary things, is another principle, and another part from the "opinionative" and "reasoning" part by which it knows contingent things. And he proves this from the principle that for those things which are "generically different, generically different parts of the soul are ordained." Now contingent and necessary are generically different, as corruptible and incorruptible. Since, therefore, necessary is the same as eternal, and temporal the same as contingent, it seems that what the Philosopher calls the "scientific" part must be the same as the higher reason, which, according to Augustine (De Trin. xii, 7) "is intent on the consideration and consultation of things eternal"; and that what the Philosopher calls the "reasoning" or "opinionative" part is the same as the lower reason, which, according to Augustine, "is intent on the disposal of temporal things." Therefore the higher reason is another power than the lower.
Objection 4. Further, Damascene says (De Fide Orth. ii) that "opinion rises from the imagination: then the mind by judging of the truth or error of the opinion discovers the truth: whence" men's (mind) "is derived from" metiendo [measuring]. "And therefore the intellect regards those things which are already subject to judgment and true decision." Therefore the opinionative power, which is the lower reason, is distinct from the mind and the intellect, by which we may understand the higher reason.
On the contrary, Augustine says (De Trin. xii, 4) that "the higher and lower reason are only distinct by their functions." Therefore they are not two powers.
I answer that, The higher and lower reason, as they are understood by Augustine, can in no way be two powers of the soul. For he says that "the higher reason is that which is intent on the contemplation and consultation of things eternal": forasmuch as in contemplation it sees them in themselves, and in consultation it takes its rules of action from them. But he calls the lower reason that which "is intent on the disposal of temporal things." Now these two--namely, eternal and temporal --are related to our knowledge in this way, that one of them is the means of knowing the other. For by way of discovery, we come through knowledge of temporal things to that of things eternal, according to the words of the Apostle (Romans 1:20), "The invisible things of God are clearly seen, being understood by the things that are made": while by way of judgment, from eternal things already known, we judge of temporal things, and according to laws of things eternal we dispose of temporal things.
But it may happen that the medium and what is attained thereby belong to different habits: as the first indemonstrable principles belong to the habit of the intellect; whereas the conclusions which we draw from them belong to the habit of science. And so it happens that from the principles of geometry we draw a conclusion in another science--for example, perspective. But the power of the reason is such that both medium and term belong to it. For the act of the reason is, as it were, a movement from one thing to another. But the same movable thing passes through the medium and reaches the end. Wherefore the higher and lower reasons are one and the same power. But according to Augustine they are distinguished by the functions of their actions, and according to their various habits: for wisdom is attributed to the higher reason, science to the lower.
Reply to Objection 1. We speak of parts, in whatever way a thing is divided. And so far as reason is divided according to its various acts, the higher and lower reason are called parts; but not because they are different powers.
Reply to Objection 2. The lower reason is said to flow from the higher, or to be ruled by it, as far as the principles made use of by the lower reason are drawn from and directed by the principles of the higher reason.
Reply to Objection 3. The "scientific" part, of which the Philosopher speaks, is not the same as the higher reason: for necessary truths are found even among temporal things, of which natural science and mathematics treat. And the "opinionative" and "ratiocinative" part is more limited than the lower reason; for it regards only things contingent. Neither must we say, without any qualification, that a power, by which the intellect knows necessary things, is distinct from a power by which it knows contingent things: because it knows both under the same objective aspect--namely, under the aspect of being and truth. Wherefore it perfectly knows necessary things which have perfect being in truth; since it penetrates to their very essence, from which it demonstrates their proper accidents. On the other hand, it knows contingent things, but imperfectly; forasmuch as they have but imperfect being and truth. Now perfect and imperfect in the action do not vary the power, but they vary the actions as to the mode of acting, and consequently the principles of the actions and the habits themselves. And therefore the Philosopher postulates two lesser parts of the soul--namely, the "scientific" and the "ratiocinative," not because they are two powers, but because they are distinct according to a different aptitude for receiving various habits, concerning the variety of which he inquires. For contingent and necessary, though differing according to their proper genera, nevertheless agree in the common aspect of being, which the intellect considers, and to which they are variously compared as perfect and imperfect.
Reply to Objection 4. That distinction given by Damascene is according to the variety of acts, not according to the variety of powers. For "opinion" signifies an act of the intellect which leans to one side of a contradiction, whilst in fear of the other. While to "judge" or "measure" [mensurare] is an act of the intellect, applying certain principles to examine propositions. From this is taken the word "mens" [mind]. Lastly, to "understand" is to adhere to the formed judgment with approval.
Source: Summa Theologica (New Advent)