Why the Mind Cannot Err With Respect to Essence

In the Summa Theologica (I, Q 85, A 6), St. Thomas argues that the intellect cannot err with respect to a thing's essence. This seems counter-intuitive, as Thomas himself wrote an entire work on properly understanding essence. So why one cannot err in apprehending essence is not entirely clear. In order to understand what is meant by his claim, we will determine what the intellect is, how it operates, what an essence is, and what is meant by error on the part of the intellect. To the question treated in the Summa, we being with the sed contra:

Augustine says (Questions. 83, qu. 32), that "everyone who is deceived, does not rightly understand that wherein he is deceived." And the Philosopher says (De Anima iii, 10), that "the intellect is always true."¹

This gives us a two-pronged approach to the problem. On one hand, there is the proposition that deception implies an imperfect operation of the intellect. On the other hand, there is the proposition that the intellect itself is true by nature. This prompts an account of exactly what the intellect is, for if there is error or deception at all, it would seem to be on account of the intellect (as is stated in objections 1 and 2 of the same question).²

The intellect is understood as being passive in that it is predisposed to what is intelligible, as Thomas explains, “[T]he human intellect, which is the lowest in the order of intelligence and most remote from the perfection of the Divine intellect, is in potentiality with regard to things intelligible, and is at first 'like a clean tablet on which nothing is written,' as the Philosopher says.”³ Aristotle likewise compares the intellect to the senses because of its passivity to external objects. But neither intellect nor sense is purely passive. Though the senses receive impressions from external objects, they are passive only in that they are potentially receptive to their proper objects.⁴ In a similar way, the passive intellect receives external objects, and is potential to them in the sense that it must lack the natures which it apprehends, as Thomas explains, “if the intellect were restricted to any particular nature, this connatural restriction would prevent it from knowing other natures.”⁵ We can see this analogously in the eye: It is incorrect to say that the eye contains light and color by its nature, rather that it is by nature receptive to light and color. Moreover, the intelligible forms which we perceive in matter exist in matter, though matter is not intelligible per se. What makes them intelligible is the power of the active intellect which discerns form from matter by abstraction.⁶ So the intellect, in order to know, must have a nature that is distinct from all the other things that it knows. Therefore, if it can know all sensible things, it must be distinct from any sensible nature. This precludes any notion of the intellect as a bodily organ, which leads us to conclude that the intellect must be something immaterial.⁷

That the intellect is immaterial helps us to understand how it is passive in one respect but active in another. While both intellect and the bodily senses are receptive to their proper objects (not in the same way but analogously), they are each active in different ways. As Thomas explains, “the difference is that sensitivity acts in the body, but the intellect acts on its own.”⁸ We can see this in the effects their proper objects have on their operation: The eye sees by receiving light, so that the material organ is effected by a material object. Too much of its object can overwhelm the organ, as when the eye is blinded by an excess of light. But the intellect is never overwhelmed by “too much truth.” Rather than being blinded or brought low by its object, the intellect is strengthened in its operation such that what was difficult to receive at first makes it see all other objects more easily.⁹ Because the intellect acquires knowledge primarily through the senses, its activity presupposes the activity of the senses. But because the intellect does not of necessity work through some bodily organ, it works by its own intrinsic principle.

As said above, the intellect is receptive to truth as a kind of potency. It acquires knowledge with reference to sensible things, but it works in itself from the self-evident first principles that it possesses by nature. And there are also principles external to a man which can aid in the acquisition of knowledge. So there are two principles involved in the activity of acquiring knowledge, extrinsic and intrinsic; extrinsic in what man learns by instruction and intrinsic in what he discovers by his own act. Because the intellect acts by an intrinsic principle, it acts on an active potency.¹⁰ Hence instruction, being an extrinsic principle, is something secondary to the activity of the intellect, and is not necessary for its perfect operation. In order to ascertain whether it is possible for the intellect to err with respect to its own operation, we should focus primarily on the intrinsic principle of knowledge.

Having given an account of the intellect, what it is and by what principle it operates, we have now to explore how the intellect understands its object. We have already mentioned that matter is not in itself intelligible but that the intellect nevertheless understands in connection with the sensitive faculties. While matter itself is not something which the intellect directly apprehends it is perceived through the senses. And it is from its perception through the senses that the intellect creates an image (a phantasm) of what is perceived from which it can abstract the intelligible form.¹¹ According to St. Thomas, one can abstract in two ways. One way is to consider something to be separate from that in which it is understood as if they were separate in reality; e.g. if one were to abstract “red” from “surface,” taking the color to be something distinct and separate from colored surfaces. This type of abstraction admits error, because the intellect in this way does not consider things as they are. The second way in which the intellect can abstract is to consider something absolutely, such that we disregard those things which are not necessary to the thing in question; e.g. if we consider “red” apart from “apple,” we are not erroneously understanding what “red” is but regarding “apple” as something unnecessary to redness. Simply put, we may correctly understand “red” by itself as an (accidental) essence, but we cannot understand it as something which exists by itself.¹²

Aristotle's insight into the process of understanding is crucial to forming a complete account of the intellect's operation. In the beginning of the Physics he explains,

[T]he natural road is from what is more familiar and clearer to us to what is clearer and better known by nature; for it is not the same things that are well known to us and well known simply. For this reason it is necessary to lead ourselves forward in this way; from what is less clear by nature but clearer to us to what is clearer and better known by nature. But the things that are first evident and clear to us are more so the ones that are jumbled together, but later the elements and beginnings become known to those who separate them out from these. Thus it is necessary to proceed from what is general to what is particular, for it is the whole that is better known by perceiving, and what is general is a kind of whole since it embraces many things as though they were parts.¹³

So, taking as the object of intellection what is perceived by the senses, the intellect begins its process of coming to know with a general and confused image of things, and from there proceeds by means of abstraction to a more precise knowledge of things in themselves. Thomas expounds on this when he says, “the perfect act of the intellect is complete knowledge, when the object is distinctly and determinately known; whereas the incomplete act is imperfect knowledge, when the object is known indistinctly, and as it were confusedly.”¹⁴ So we refer to the act of the intellect from its active potency as being perfected in the complete and determinate knowledge of its object. But this process is not possible without having a starting point which is found in the first principles.¹⁵ Yet how these first principles are acquired must be explained in order to complete our account of the process of coming to know.

According to Aristotle, the first principles required for knowledge are neither innate, for infants would be as capable of knowledge as a grown man, nor are they acquired at some later point, for learning and discovery would be impossible until then. Because knowledge is acquired through sense-perception, he concludes that the first principles are gathered from sense-perception and the accumulation of memory which, over time, make up experience. Irrational animals have the capacity of both sense-perception and memory, but they are not capable of gathering the principles of knowledge which is a capacity of the intellectual soul.¹⁶ Thomas elaborates on this by reference to the agent intellect, reminding us that the intellect is not merely passive but must actively derive the universal from the experience of particulars.¹⁷

We are now equipped to give a complete account of the intellectual operation in man. A man begins by perceiving sensible objects through his senses (rust, a hard surface, heaviness, etc.). The objects of his senses are unintelligible, but his passive intellect receives an image of the thing perceived and holds this in memory. After many instances of this, the agent intellect is able to abstract from his experience a universal principle (the concept of iron). With the universal now in mind, he is able to proceed to knowledge of particulars (knowing what iron is in general, he can identify particular bodies of iron). It is clear from this that the intellect understands its object by combination and division.¹⁸ This is easily seen with respect to the twofold composition of material things. In one sense a thing is composed of form and matter, in another sense it is composed of substance and accidents. The intellect divides with respect to both these compositions, as our very consideration of them attests, and in combination we acknowledge those composites to be whole in reality.¹⁹

With the intellect and its operations accounted for, we now consider what essence is. Aristotle explains, “a thing's essence is what a thing is.”²⁰ An essence is like a definition, and is indeed expressed by a definition, but is strictly speaking a definition of a thing in its own right, rather than a property or something which can only exist parasitically on something else: E.g. 'man' or 'tree' is an essence but 'white' is not, because 'white' only exists as a property which adheres to man or tree. There are however essential properties of man which are not accidental. “Bipedal”, for instance, is an essential part of 'man'. There is a correspondence here to species, as 'man' is a species of the genus 'animal', and one of the differentiae which distinguishes 'man' from 'cow' is “bipedal,” though “bipedal” itself is not an essence. Aristotle thus concludes, “essence will not be found in any of those things which are not a species of a genus.”²¹

To understand an essence is the same as to identify it. That is, the question, “why is that a man” can be answered by explaining what a man is, and vice versa.²² So, in a general sense, an essence is what a thing is. Depending on what is meant by a “thing” however, there can be a distinction between an essence and a thing. For example, a man is a composite of form and matter. Thus the question “what is a man?” cannot be answered without reference to a body and its constituents. To the question “why is that body a man's body?” the answer will refer to the form which makes the body a man's body. The distinction between the composite and the components points to the substantial form which composes the elements into a whole: E.g. Even though a man by definition has body parts, a collection of body parts is not a man.²³

Essence, in the formulation of its definition, is generally understood with regard to species (though the two terms are not strictly synonymous). A species is a member of some genus which is distinguished from other species of that genus by some difference.; for instance, 'man' is defined as a 'rational animal'. He is generically an animal and specifically rational, 'rational' being the key difference from other animals. It is important to note that man is distinct from animals in species, but not in genus. Man is an animal, albeit a rational one.²⁴ Though there are many men in the species 'man', the species itself is one, as Thomas says,

“[I]t is evident that a multiplicity of parts in a definition does not signify a multiplicity of essential parts of which the essence is constituted as if they were distinct things; but all signify one thing which is made determinate by an ultimate difference. It is also evident from this that there is one substantial form for every species.”²⁵

An essence is expressed by a definition, so that there are many men does not prove that there are many essences or species of man, because where and how many they are does not belong to the definition of man. Neither does the multiplicity of “parts” implicit in the definition (for example man's two legs) mean that there are many essences within an essence, because essence belongs primarily to substance, but each of those so-called parts do not exist as a substance.²⁶ So one could say that there is a definition of “leg” and thus that a leg has in a certain sense an essence, but a leg is not a substance. Furthermore, a leg is defined as part of a body, so it cannot be a leg unless it is subsumed under the essence of the thing to which it belongs.

What we have considered thus far is essence in terms of its definition. But we should consider essence in terms of being as well. As regards to what a thing is in itself, essence is the composite of form and matter.²⁷ It is by its essence that an individual thing participates in being. This is not to say that everything which has an essence also has being, but that all things which exist do so insofar as they have an essence. As noted above, it belongs to the species of material beings to have matter, but not this or that matter. The prerequisite matter for a given species is called indeterminate matter, while the matter by which an individual participates in the species is called determinate. Therefore material things in particular participate in a species through determinate matter and, by the same token, are distinct from other individuals who share the same essence.²⁸ Though essence is the composition of form and matter, neither the form nor the matter are separate in reality, this means that essence cannot be broken into parts: It is understood by combining terms (form and matter, genus and difference, etc.) into a definition, but an essence is a distinct unity in reality.

Now that it is clear what the intellect is and how it operates, as well as what essence is and how it is known, we may at last consider Thomas' argument in full. He recalls the comparison between sense and intellect, viz. that both are always directed to their proper object, adding that, “as long as the faculty exists, its judgment concerning its own proper object does not fail.”²⁹ The senses may be deceived with respect objects that are “common,” such as size or color; e.g. the eye perceives the sun to be yellow and an inch across. The senses may also be deceived with respect to objects in an accidental way; e.g. salt may be mistaken for sugar. With common objects, the eye is deceived because what is perceives is other than what it is, as the sun is not yellow nor an inch across. With the latter case (salt and sugar), the eye is not deceived with respect to what it apprehends, for it takes the salt to be white and translucent, but is deceived in conflating its object with what appears to be an identical object but is in fact different. In both of these cases, one's sight functions perfectly; it apprehends precisely what it receives. This is why Thomas says, “as long as the faculty exists, its judgment concerning its own proper object does not fail.”³⁰

The proper object of the intellect is essence (in this instance referred to as quiddity which simply emphasizes the “what” of the thing). As we have said, a thing's essence is what a thing is, and it is something unified which we come to understand as a composition. So, in an absolute sense, we have an essence that is indivisible in reality which is apprehended by the intellect which is immaterial. But in order for the intellect to understand an essence, it must move to its act of knowing by means of combination and division. Thus we have two types of error to consider, the apprehension of an object by the intellect taken absolutely, and the understanding of it in terms of its definition (its intellectual composition). If the intellect is said to err with regard to apprehending something absolutely, as for instance one might be said to err in his understanding of what “space” is, then he does not know what it is. There is nothing complex about this type of “error.” Either one knows it or he does not. In fact, if one does not know “space” then he is not mistaken about it, because there is nothing in his mind to mistake. In this regard, i.e. with things that are apprehended simply, the mind cannot err. It is only in the process of discerning composition, as it pertains to a thing's definition, that the mind can be mistaken; for instance, if one were to regard a circle as a “three-sided figure.” Another way in which the intellect could err is to combine incompatible terms into a definition, as for instance in “rational plant.” The error in both of these examples is attributed either to the mis-attribution of a definition or to the combination of incompatible terms into an attempted definition.³¹ But both of these errors are accidental to intellectual operation. The result of both of these errors is to fail to understand an essence completely.

An analogous way of understanding the truth of intellection is by considering the various levels of “error” with regard to apprehension. For example, a young reader may come across a word never before seen and pronounce it “hoar-is-on.” The mistake is in combining the syllables in a way that misses the meaning of the word. The is no “half-understanding” in this regard; the word is a complete mystery. With help from his teacher, the child learns that the word is “horizon” and suddenly understands completely. In another example, a hunter might mistake an elk for a deer. He does not misapprehend the essence of elk, rather he completely understands the essence of deer and misappropriates it to the wrong sense object. Finally, in a more complex case, one might attempt to define an animal without reference to form or the soul, regarding it as merely mechanical or “purely material.” In this case, it should be said that either he does not understand what an animal is because, while he may understand the parts of its definition, i.e. the terms “matter,” “body,” “life,” “motion,” etc. are correctly apprehended, but they are out placed out of order in that definition. So he does not “half-understand” what an animal is; he does not understand what an animal is at all, though he understands all of the parts necessary to combine a correct definition and come to knowledge of its essence.

It may seem trivial to argue for the truth of the intellect if what is meant is simply that error is only in the process of learning and that substantial ignorance is absolute. But all science (especially philosophy) is at stake if this distinction is not understood properly. The intellect has the power to know the truth, it has the means to attain it, and it knows truth with certainty. That the means of attaining knowledge is prone to fault does not mean that the intellect does not know truth, for to say that the intellect is not true is to say that there is no intellect.

1 ST I, Q 85, A 6 sed contra.(Benziger Bros., 1947)

2 Op. cit. obj 1 & 2

3 ST I, Q 79, A 2 resp.

4 Aquinas, Commentary on De Anima, trans. Foster and Humphries (New Haven: Yale University Press, 1951) 676.

5 Op. cit. 680.

6 ST I, Q 79, A 3 resp.

7 Aquinas, Commentary on De Anima, 681.

8 Op. cit. 688.

9 Ibid.

10 St. Thomas Aquinas, On the Teacher in Thomas Aquinas: Selected Writings, trans. Ralph McInerny (New York, NY: Penguin Books, 1998), 199.

11 ST I, Q 86, A 1 sed contra

12 ST I, Q 86, A 1 resp.

13 Aristotle, Physics in Aristotle’s Physics: A Guided Study, trans. Joe Sachs (New Brunswick: Rutgers University Press, 2011), 33 (1.i).

14 ST I, Q 85, A 3 resp.

15 Physics 99b20

16 Physics 100a4

17 St. Thomas Aquinas, Commentary on the Posterior Analytics, trans. Fabian R. Larcher, (http://dhspriory.org/thomas/PostAnalytica.htm), II, xx.

18 ST I, Q 85, A 5 resp.

19 ST I, Q 85, A 5, ad 3

20 St. Thomas Aquinas, Commentary on the Metaphysics, trans. John P. Rowan, (Chicago, 1961) (http://dhspriory.org/thomas/Metaphysics7.htm), VII, iii, 581.

21 Ibid.

22 Aristotle, Metaphysics in Commentary on the Metaphysics by Thomas Aquinas, trans. John P. Rowan (Chicago, 1961) (http://dhspriory.org/thomas/Metaphysics7.htm), 1651.

23 Commentary on Metaphysics, 1674.

24 Aquinas, Commentary on Metaphysics, 1328.

25 Aquinas, Commentary on Metaphysics, 1339.

26 Op. cit., 1339.

27 Thomas Aquinas, De Ente et Essentia, trans. Armand Maurer (Toronto: Pontifical Institute of Mediaeval Studies, 1968), 36 (IV.iii).

28 De Ente, IV.v

29 ST I, Q 85, A 6 resp.

30 ibid.

31 ST I, Q 85, A 6