Rational Knowledge (Intellect)

Intellect and Reason: What is unique and distinctive of human beings.

Intellect is the general power of the soul which sets humans apart from animals.  It is manifest in activities which are unique and exclusive to humans.  The making of tools is an exclusively human prerogative.  While animals such as apes and some birds use tools, only human beings construct them of various parts, and keep or save them for later use.  The construction of tools is very different from their use.  The making of a tool presupposes

  • (a) a knowledge of the end or purpose as an end
  • (b) a knowledge of a means to an end, and
  • (c) a knowledge of the relationship between the end and the means. 

This entails abstract knowledge, for the tool is constructed to be used on many occasions, and there always exists a relation between the purpose or end in the abstract and the tool which is the required as a means for achieving that end.

The invention of languages is another peculiarly human activity.  It is true that other animals communicate through sounds, motions, perhaps even scents, but such methods of communication is quite limited, being restricted to reporting threats or desires in the here and now.  Human language is unique in the we use sounds (and eventually gestures and marks on stone or paper) as conventional or artificially devised symbols to express thoughts, especially abstract thoughts which transcend the here and now, and refer to things past or future, distant in space, or universal in scope and content.  My cat often meows to signify her hunger, and her desire that I fill her food dish, but she can’t tell me that food is good and she enjoys eating it when she is not actually eating it.

These phenomena serve as the bases of further purely human activities such as religion, art, poetry, morality and culture.  Thus we readily see that the human being possesses a cognitive power which is superior to the cognitive powers of animals.  This superior cognitive power is called intellect.

Nature of the Intellect.

While we may readily admit that the intellect is superior to the cognitive powers of animals, i.e., the senses, humans nevertheless possess the senses, too.  So we must inquire how the intellect differs from the senses, for the difference in the intellect’s operation will indicate the difference in the nature of intellect, and so human nature, relative to animals which lack this power. Likewise, the senses are in a certain sense immaterial, but the intellect seems to be immaterial to a greater extent. (Perhaps one should say it is material to an even lesser degree than the senses.)  This will in turn illumine the question of how, and to what extent, the intellect, and the soul of which it is a power, is able to operate, and so exist, without any material part of the human body.

A hallmark of the philosophy of Saint Thomas Aquinas, like Aristotle’s on which it builds, is to define what possesses a power, in this case the rational soul of humans, by the power or powers it possesses, in this case the intellect, and powers are in turn defined by their acts, and act by their objects. Just as an animal possessing sight is able to see, seeing in turn is defined by the act of grasping or becoming one with the object of sight, color (primarily) and shape and motion (secondarily, by means of color). So, the intellect as a knowing or cognitive power, is defined by its objects – common and proper:

A. The material object of the intellect.

By the material object of the intellect, we mean anything which in any way whatsoever is knowable by the cognitive power. The material object of the intellect is constituted by all kinds of beings – real entities as well as logical entities, substances as well as accidents, bodies as well as spirits. All these beings are included in the material object of the intellect.

B The formal object of the intellect.

The formal object of the intellect is twofold– the common formal object, and the proper formal object. The common formal object is concerned with the intellect in itself insofar as it is an intel­lect. The proper formal object is concerned with the intellect as united to the body, i.e., with the in­tellect in its present human condition.

1. The common formal object of the intellect.

Since whatever is or can be is intelligible, whatever is or can be is the formal object of the intellect as such. Consequently, it is being that is the common object of the intellect:

The object of the intellect is universal being or universal truth.

ST I, 55, 1.
2. The proper object of the intellect.

Our intellect, as united to the body, cannot know all things, because it takes its objects from the senses, and is thus unable to know directly that which cannot be apprehended by the senses. The di­rect and primary formal object of the intellect united to the body is the abstract essences of sensible things. Our intellect understands other things in relation to this object.

The object of the intellect is something universal, namely, being and truth. The first object of our understanding in this state of life, however, is not every being and every truth, but being and truth as considered in material things, from which it acquires knowledge of all other things.

ST I, 87, 3, ad 1.

The proper object of the human intellect which is united to a body is a quiddity or nature ex­isting in corporeal matter; and through such natures of visible things, it rises to a certain knowledge of things invisible…

ST I, 84, 7.

And this is so because the object of the intellect depends upon the object of the senses, and the object of the senses are the accidents of material beings. Hence only the abstract essences of material beings are the primary object of our intellect.

C. The knowledge of singular material objects.

The singular, object cannot be known directly by our intellect, but by the senses. And this is because the principle of individuation in material beings is individual prime matter, which is, as such, unintelligible. Of course, the individual is not unknown because it is individual, but because it is ma­terial.

Intelligibility is incompatible with the singular not as such, but as material, for nothing can be understood otherwise than immaterially. Therefore if there is an immaterial singular such as the in­tellect, there is no reason why it should not be intelligible.

ST I, 86, 1 ad 3.

Our intellect cannot know the singular in material things directly and primarily. The reason for this is that the principle of singularity in material things is individual matter, whereas our intellect … understands by abstracting the intelligible species from such matter. Now what is abstracted from individual matter is the universal. Hence our intellect knows directly only the universal. But indirectly, and as it were by a kind of reflection, it can know the singular, be­cause…even after abstracting the intelligible species, the intellect, in order to understand, needs to turn to the phantasms in which it understands the species… Therefore it understands the universal directly through the intelligible species, and indirectly, the singular represented by the phantasm.”

ST I, 86, 1.
D. Suprasensible reality.

Suprasensible reality is only known in relation to sensible beings, because immaterial realities cannot be presented to our intellect by proper species inasmuch as the intellect depends on the senses. Nevertheless, our intellect attains an indirect and imperfect knowledge of them. Through the knowledge of material beings, the intellect can be led to affirm the existence of immaterial beings, such as God.

By reasoning from material reality, the intellect is able to recognize God as first cause. The perfections of creatures are therefore attributed to God insofar as he is the cause of creatures (the way of causality). As the first cause, however, God can have none of the imperfections of creatures which are due to their limited natures. Such imperfections must therefore be denied of God or removed from him (the way of negation or remotion). Thus, we say that God is infinite, immaterial, etc. Those perfec­tions of creatures, however, which imply no imperfection in their proper concepts may be attributed to God in an analogous and eminent way (the way of eminence). Thus, we say that God is good, wise, etc. In any statement we make about God, however, all three of these ways are at work, for we always speak of God from creatures which are better known to us, and we recognize him as their cause who is free from all im­perfection and possesses all perfection in an eminent way. The fact that our knowledge of God is imper­fect is due not to the divine nature (which is perfectly intelligible in itself), but to limitations of our intellect which depends on material things represented by phantasms. See ST I, QQ.12 and 13; Q.88, aa.1 and 2. of: Sent. I, d.3, Q.1, a.1, div. text.

The Acts of the Intellect

There are three acts of the Intellect, acts by which we grasp, arrange and infer or come to understand the proper objects of our intellectual knowledge, the quiddity or universal essences of material things, primarily, and other immaterial or abstract objects through them.

1. Simple apprehension. Simple apprehension represents the simple intellectual vision of one essence, even if this vision is general and vague.

2. Judgment. The intellect says whether or not that which is seen in simple apprehension corresponds to reality. Truth, which is defined as the conformity between intellect and reality, lies in the judgment.

3. Reasoning. Reasoning is the process by which the intellect goes from the understanding of one truth to another truth which is implicitly contained in the first.


These three are the basic acts of all intellectual thinking and reasoning, and they are all used in, and oriented toward, understanding the quiddities of material things, i.e., universal essences. They are also used in the initial grasp of every and all quiddities, as these are gleaned and refined from our experience of the world through our senses. Aquinas made one of his greatest contributions to philosophy in describing in some detail this process of gleaning and refining essences from sense experience, which he learned from Aristotle.

1. Dependence of the intellect upon phantasms.

The ideas of the intellect depend upon the internal senses, for in Aristotelian philosophy, it is said that “nothing is in the intellect that was not first in the senses.” The intellect is like a black­board (tabula rasa) on which nothing is written. This dependence is evident in various ways. First, we might note that when the imagination is disturbed (e.g., by a brain injury), the intellect is disturbed too. Second, we note that we have no ideas of objects whose sensible qualities are unknown. (A blind man has no idea of color). Third, it is a fact of experience that the intellect acts always in depen­dence upon the imagination. The intellect and the internal senses operate simultaneously:

The Philosopher says (De anima III, 7), “the soul understands nothing without a phantasm.” When we try to understand something, we form certain phantasms to serve us by way of example, in which, as it were, we examine what we are desirous of understanding. For this reason, it is that when we wish to help someone to understand something, we lay examples before him, from which he forms phantasms for the purpose of understanding… .

ST I, 84, 7

This is important in psychiatry; the disturbances of mental patients are not due to the intellect as such, but to the appetitive part of man, the passions; or to the disturbances of man’s internal sen­ses. That is why biochemistry is so useful in psychiatry. It changes the disposition of the body – in this case, the brain.

2. The agent intellect.

a. Existence of the agent intellect.

The agent intellect is absolutely necessary. Because the phantasms are concrete and our ideas are universal and abstracts there must be a power which abstracts intelligible species from the phan­tasms. We call this power the “agent intellect.” Without the action of the agent intellect, phantasms are only potentially intelligible.

According to the opinion of Plato, there is no need for an active intellect in order to make things actually intelligible… For Plato supposed that the forms of natural things subsisted apart from matter, and consequently that they are intelligible: since a thing is actually intel­ligible from the very fact that it is immaterial… But since Aristotle did not allow that forms of natural things exist apart from matter, and since forms existing in matter are not actually intelligible; it follows that the natures or forms of sensible things which we understand are not actually intelligible. Now nothing is reduced from potentiality to act except by something in act; as the senses are made actual by what is actually sensible. We must therefore assign on the part of the intellect some power to make things, actually intelligible, by abstraction of the species from material conditions. And such is the necessity for an active intellect.

ST I, 79, 3, co.
b. Functions of the agent intellect.

Not only does the active intellect throw light on ‘illuminate’ the phantasms: it does more; by its own power it abstracts the intelligible species from the phantasm. It throws light on the phantasm, because, just as the sensitive part acquires a greater power by its conjunction with the intellectual part, so by the power of the active intellect, the phantasms are made more fit for the abstraction therefrom of intelligible intentions. Furthermore, the active intellect, ab­stracts the intelligible species from the phantasm, forasmuch as by the power of the active intellect we are able to disregard the conditions of individuality, and to take into our consideration the specific nature, the image of which informs the passive intellect.

ST I, 85, 1, ad 4.

The distinction of active and passive intellect in us is in relation to the phantasms, which are compared to the passive intellect as color to the sight; but to the active intellect as colors to the light, as is clear from De anima III.

ST I 54, 4 sc. Cf. In de anima III, lect. 10, n. 728, ff.
c. The agent intellect and the phantasm as causes of intellection.

The agent intellect and the phantasm “together” produce the intelligible species which actuates the potential intellect. But the agent intellect is the principle efficient cause since it is the cause of its immateriality. But it always acts in the same way and therefore cannot alone ex­plain why the species is of a determinate nature. The phantasm, on the other hand, can explain why the species is of a determinate nature, not why the species is immaterial. Therefore both the phantasm and the agent intellect are the cause of the intelligible species.

The phantasm, however, is an instrumental cause employed by the agent intellect. Because the phantasm is material, it cannot act by virtue of its own power upon the immaterial intellect.

In the reception through which the possible intellect receives species from phantasms, the phan­tasms act as instrumental and secondary agents. The agent intellect acts as the principle and first agent. Therefore, the effect of the action is received in the possible intellect according to the conditions of both, and not according to the conditions of either one alone. Therefore, the possible intellect receives forms whose actual intelligibility is due to the power of the agent intellect, but whose determinate likeness to things is due to cognition of the phantasms. These actually intelligible forms do not, of themselves exist either in the imagination or in the agent intellect, but only in the possible intellect.

De Ver. 10, 6 ad 7.

3. The potential (possible) intellect.

Once the species is ready and it is received by the potential intellect, the potential intellect elicits the act of understanding. It is the potential intellect which, properly speaking, is said to un­derstand. The proper act of the agent intellect is to dematerialize phantasms by stripping them of their material conditions and presenting their essences to the potential intellect.

The potential intellect is in potency, for it needs to be complemented by the intelligible species before being fully able to elicit its act of intellection. For this reason, it is called a pas­sive potency. The agent intellect is always ready to act and by its action changes the phantasm; hence it is said to be an active potency. The agent intellect, however, is active in the sense that it elicits its act at once when a phantasm is present. Unlike the passive–intellect, which has no actions until after it has been reduced to act, the action of the agent intellect accompanies it instantly, since the very nature of the agent intellect consists in act. See ST I, 54, 1 ad 1

The agent intellect and the potential intellect are two really different powers:

There is a distinction between the power of the active intellect and of the passive intellect: because as regards the same object, the active power which makes the object to be in act must be distinct from the passive power, which is moved by the object existing in act. Thus the active power is compared to its object as a being in act is to a being in potentiality; whereas the passive power, on the contrary, is compared to its object as being in potentiality is to a being in act.

ST I, 79, 7 co.

When the species is in the intellect, the potential intellect elicits its act of understanding. In the act of understanding, the-intellect produces the mental word called the expressed species. In this species, the intellect knows the external object. The expressed species is really distinct from the object which is understood through that species. The concept is merely a means in which the real object is known.

4. Intellectual memory.

Intellectual memory means the capacity of our intellect to retain intellectual knowledge; e.g., we retain the concept of a tree, which appears explicitly whenever we wish to have the explicit knowledge of a tree.

Intellect, Immateriality, and the Spiritual Nature of the Human

First, we must clarify the meaning of the term “immaterial.” We have already seen that immateri­ality is the root of knowledge. Such immateriality implies a certain independence from matter, which per­mits the cognitive power to receive the forms of other things by way of intentions (as distinct from re­ceiving such forms in a substantial or material way). Obviously, the intellect is immaterial in this -sense. But the immateriality predicated of the intellect implies also a spirituality, i.e., an intrinsic independence of matter in operation and being. The operation of the intellect is not exercised in any organ but springs from a spiritual principle. It is in this sense that the term “immaterial” should be understood when there is a question of the immateriality which corresponds to the intellect.

It is far from easy to prove the spirituality of the intellect. Cajetan says that we must accept this by faith, not by reason, for it is impossible to prove it. This, however, is not Aquinas’ conten­tion, nor that of his other commentators. The arguments proving the spirituality of the intellect have to be based on its operation. If the operation of the intellect transcends matter, then the intellect is spiritual. The operation follows the being.

A. Arguments for the spiritual nature of the intellect.

1. The argument from the operation of the intellect.

The intellect knows the universal. All concepts are universal.

  • a) Some are universal concepts of concrete things, like “tree,” “man,” “animal,” etc.
  • b) Some are of abstract things, like “humanity,” “truth,” “liberty,” “principle”, etc.
  • c) Some are of immaterial things “substance,” or “spirit.”

To show that our knowledge, in virtue of its universal concepts, is an immaterial operation transcending the conditions of matter, we must first consider the following conditions of matter:

  1. MOTION.
    • a. Substantial change, which is possible because of the existence of prime matter.
    • b. Local motion, which is due to the existence of place, which is in turn a consequence of the extension of bodies.
  2. PLACE. Place exists because of the quantitative nature of bodies (their extension).
  3. TIME. Time, as the measure of motion with respect to before and after, follows upon motion.
  4. INDIVIDUALITY. Individuality follows in that prime matter and quantity are the principles of individuation of mobile things.
  5. QUANTITY. Quantity is the first accident which follows upon prime matter.

The universal concept abstracts from all these conditions. Concepts are not subject to motion. They abstract from place and motion, and they are not individual.

The principle of diversity among individuals of the same species is the division of matter according to quantity. The form of this fire does not differ from the form of that fire, except by the fact of its presence in different parts into which the matter is divided; nor is this brought about in any other way than by the division of quantity– without which substance is indivisible. Now, that which is received into a body is received into it according to the division of quantity. Therefore, it is only as individuated that a form is received into a body. If then the intellect were a body, the intelligible forms of things would not be received into it except as individuated. But the intellect understands things by those forms of theirs which it has in its possession. so, if it were a body, it would not be cognizant of universals, but only of particulars.

SCG II, c. 49, no. 4.
2. Arguments through self-reflection.

Reflection may be of two kinds.

  1. IMPROPER: when a subject knows the acts of a cognitive power by means of another cog­nitive power; e.g., the common sense knows the act of the external senses, and the intel­lect knows the acts of the internal senses.
  2. PROPER: when a cognitive power is formally conscious of its own acts and itself. The term “formally” indicates that we are not concerned with the implicit consciousness that accompanies every act of understanding, but with the cognitive activity in which the in­tellect takes itself and its actions as objects of its considerations. In other words, the intellect is conscious of its own operation. It knows that it is knowing.

In proper reflection, a cognitive potency knows itself and its act, so that the principle and term or object of the operation are identical. Identity excludes any intermediary, for nothing can come between a thing and itself. But in an organic potency, the object is received through the organ, so that the organ is between the potency and the object. Moreover, an organ is some­thing material and extended, having parts outside of parts. But in self-reflection, the whole applies itself to (reflects upon) the whole, which is impossible for extended reality… Therefore, a cognitive potency which is self-reflective cannot be organic.

H. J. Koren, Introduction, p.164.

The action of no body is self-reflexive. For it is proved in the Physics that no body is moved by itself except with respect to a part, so that one part of it is the mover and the other the moved. But in acting, the intellect reflects on itself, not only as to a part, but as to the whole of itself.

SCG II, c.49, no. 8.

The soul reflects upon its own acts and knows that these acts exist. A power which uses a corporeal organ, however, could not reflect upon its own proper act since the instrument by which it knows itself as knowing would be between the power and the instrument by which it knows directly.:. The intellect, however, since it does not use a corporeal organ, can know its act; inasmucb6 as it is affected (patitur) in a way by the object and is informed by the species of the object.

Sent. III, d. 23, Q. 1, a. 2, ad 3.

The intellect knows itself. This cannot happen in any power whose operation is through a cor­poreal organ. The reason for this is, as Avicenna explains, “for any power operating through a corporeal organ, it is necessary that the organ be an instrument ‘or means4 between that power and its object.” Thus, the sense of sight knows nothing except those things whose species can be in the eye. ‘The physical organ, the eye, is a means or instrument between the power of sight and the object seen (the tree).’ Since it is not possible that a corporeal organ be between any power and the very essence of that power ‘since these are identical’, it is impossible for any power operating by means of a corporeal organ to know itself.

Sent. II, d. 19, Q. 1, a. 1. Cf. De ver. Q. 1, a. 9; SCG II, c. 50; ST I, 75, 5.

B. Consequences of the immateriality of the intellect.

Intellection is an act of the soul alone in which the body has no part. The soul is the sole re­mote means by which man understands. Hence, since the intellect is inorganic, i.e., it has no organ, no part of the body can be called the seat of the intellect. The intellect does not exist in the brain, although the intellect depends upon the internal senses which supply the intellect with the object of understanding. We don’t think with our brains. Nevertheless, it must be admitted that the brain, which is the seat of the inter­nal senses, is more intimately connected with the activity of intellect tin the rest of the body. For this reason, the brain may be called the extrinsic organ of the intellect.

C. The individuality of the intellect in each human person.

Averroes, the Arabian commentator on Aristotle, asserted that there is only one intellect for all human beings. Aquinas maintained that each human being has his/her own intellect. From our internal ex­perience, we can know that thinking and understanding are activities of individuals… “I think, there­fore I am,” Descartes said. Theoretically speaking, however, the problem is not so easy. For the principle of individuation in spiritual substances is the form, and it is difficult to see how something spiritual can be multiplied numerically. On how multiplicity may be found in non-material things, see ST I, 30, 3; I, 50, 3. For Thomas’ arguments for the individuality of the human intellect, see ST I, 76, 2; SCG II, c.73 & 75. On how the multiplicity of human souls involves the relationship between soul and body, see ST I, 76, 2, ad.1 and ad.2.

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