The Unity of the Human Person

In ScG, II.56–72, St. Thomas embarks upon a consideration of intellectual substances which are united to bodies, later considering those which are not thus united (angels, see ScG, II.91ff). In these chapters, then, we will follow St. Thomas in staking out the middle ground between a materialist account of the human person (ruled out by the spiritual character of the intellectual soul) and a dualist version, which was then, as now, popular to defend, especially in theological circles.

  1. an overview of these chapters;
  2. brief summary of main points;
  3. various questions;
  4. a brief recap of the positions excluded, and why;
  5. the arguments that an intellectual substance can be the form of a body;
  6. the refutation of difficulties against this view;
  7. clarifications about the mode of union;
  8. concluding questions.

6.1. An Outline of ScG, II.56–72

St. Thomas’s treatment here might be divided as follows:

In the above outline, we can also note how the subsequent week’s reading adds certain crucial topics.

6.2. Summary of the Account of the Human Person’s Unity

In ScG, II.56, St. Thomas gives various options for ways in which the intellectual substance might be united to a body. (An aside: Did the arguments in ScG, II.46 prove that for the perfection of the universe there must be an intellectual substance united to a body? Or just intellectual substances? Does Aquinas need another argument for this? Or is our case a bit more obvious?)

Here they are:

  • Physical, quantitative contact
  • Physical, qualitative contact, or virtual contact (contact of power)
  • The difficulty, however, is that contact of power is unity in action, not unity simply
    • One simply by indivisibility: this cannot work because intellectual soul and body are two
    • One simply by continuity: this cannot work because the intellectual soul is non-bodily
    • One simply in ratione, or in account or nature
      • This seem impossible for five different reasons …

Note that St. Thomas does not think that this sort of contact is useless for explaining psychological-physiological interaction, merely that it is not the fundamental reason: see ST, Ia, q. 76, a. 6, ad 3: “A spiritual substance which is united to a body as its motor only, is united thereto by power or virtue. But the intellectual soul is united by its very being to the body as a form; and yet it guides and moves the body by its power and virtue.”

So, by the end of the above, it seems that there is no way in which the human intellectual soul could be truly united to a body. Yet “man’s very nature seemed to controvert this opinion,” and so what Plato others are trying to do in subsequent chapters is thereby explained: “They devised certain solutions so as to save the nature of man.” (II.57)

In St. Thomas’s treatment of the human person’s unity, one can analyze the various points of the philosophy of nature and metaphysics of form which he brings to bear. However, one could also note the ways in which St. Thomas appeals to experience as a basis for argument:

  • II.57: “… there are nevertheless certain operations common to [soul] and the body, such as fear, anger, sensation, and so forth; for these happen by reason of a certain transmutation in a determinate part of the body, which proves that they are operations of the soul and body together.”
  • II.58: “Now we find that the various actions of the soul hinder one another, since when one is intense, another is remiss. It follows, then, that these actions, and the forces [vires] that are their proximate principles, must be reduced to one principle.”
  • II.59: “Now it is clear that it is properly and truly said that man understands, for we would not inquire into the nature of the intellect except for the fact that we understand ourselves.”
  • II.60: “Now man’s will is not outside man, as though it were vested in a separate substance, but is in man himself. Else he would not be master of his own actions, for he would be acted upon by the will of a separate substance … But this is impossible, and would do away with all moral philosophy and political life.”
  • II.60: “If, therefore, the possible intellect understands separate substances, we also understand them. Yet this is clearly untrue, for we stand in relation to them as the eye of the owl to the sun, …”

Aquinas also combats various views that are essentially too materialist, and appeals to behaviors, changes, or experiences which exceed the capacities of mere matter.

  • II.62: “Now even the operation of the nutritive soul exceeds the power of the elemental qualities … Consequently, the vegetative soul cannot be produced by the mixture of the elements, and much less, therefore, the sense and possible intellect.”
  • II.63: “The temperament does not move an animal’s body by local movement: for it would follow the movement of the predominant element, and thus would always be moved downwards. But the soul moves the body in all directions. … The soul rules the body, and curbs the passions that result from the temperament.”
  • II.65: “Moreover, many men were moved to take up this position through believing that there is nothing that is not a body, being unable to transcend their imagination which is only about bodies.”

The arguments that the soul is not a body, or at the level of sensation only, or even the imagination begin to appeal to various activities which one can do (viz., the intellect) but the other cannot, or some property of one which does not belong to the other.

6.3. Some Intervening Questions

Our main question for next time will be to follow St. Thomas as he makes good on his promise to show how the human intellectual soul is united to the body as its form. Note the overarching “negative induction” with which he begins ScG, II.68:

Accordingly, from the foregoing arguments we are able to conclude that an intellectual substance can be united to the body as its form. 

  • For if an intellectual substance is not united to the body merely as its mover, as Plato stated, 
  • nor is in contact with it merely by the phantasms, as Averroes held, but as its form; 
  • and if the intellect by which man understands is not a preparedness in human nature (as Alexander maintained), nor the temperament (as Galen said), nor harmony (according to Empedocles), nor a body, the senses, or imagination (as the ancients asserted), 
  • it follows that the human soul is an intellectual substance united to the body as its form. This can be made evident as follows.

Apparently, St. Thomas is advancing an argument by elimination. After all the other positions, his is the only one left. Is this negative induction complete? Are there other options? Perhaps:

  • The intellect is either part of our being or not
    • If not part of our being, we must be in contact with it somehow (Averroes)
    • If intellect is part of our being, then either it is a thing or a cause
      • If a thing, either an immaterial thing and joined to the body as agent in operation (Plato)
      • Or some sort of a material thing (Alexander, Galen, et al.)
    • Otherwise, it is a cause, namely, an immaterial co-cause with matter of a physical being, and the leading candidate here is form (Aristotle, Aquinas).

Aquinas offers two conditions for the last option (“For one thing to be another’s substantial form …”) and then offers two series of arguments to complete the chapter, one from “the wondrous connection of things,” and another from a consideration of the hierarchy among things. Is the reason why the intellectual soul joined to a body found in this chapter or others? Is there a final cause for this union?

And yet, since the human soul’s act of intelligence needs powers (namely, imagination and sense) which operate through corporeal organs, this by itself shows that the soul is naturally united to the body in order to complete the human species.

How does this positive argument work?

St. Thomas, engaged in an extensive dialectical campaign to “save human nature” with its true philosophical account. One commentator notes:

As Josef Pieper puts it, “Aristotle is for Thomas (in the measure in which he follows him) nothing more nor less than a clear mirror of the natural reality of creation.” . . . Thomas refines and develops his position in and through dialectical encounters with the received opinions of Plato, Averroës, Origen, Aristotle, and others. The text is not fruitfully read as a systematic or demonstrative treatise, but rather as a dialectical inquiry, which is driven by the attempt to come to terms with a set of difficulties.

Hibbs, Dialectic and Narrative in Aquinas, 80.

Indeed, just as Plato was described in ScG, II.57, so also “Thomas himself embraces the project of saving the nature of man. The second book enacts a dialectical project of saving the phenomena.” However, this is not to deny the value of demonstrative reasoning, but to put it in its natural place, because we do not automatically stumble upon demonstrations.

It is to say that the dialectical path to the principles, not the demonstrative route from principles to conclusions, is preeminent in the second book of the Contra Gentiles. Thomas’s engagement of all pertinent authorities, of authorities from various traditions, is an exemplary instance of what Alasdair MacIntyre calls tradition-constituted inquiry. The conception of inquiry to which tradition-constituted inquiry is most antithetical is, of course, that of Descartes, who sought to stand outside of all traditions, indeed outside of human language, to achieve clear and distinct ideas.

Ibid., 81.

Why is this mode of inquiry opposed to Descartes? This of how Aquinas approaches his interlocutors, who they are, how they are arranged against him, how seriously they are taken, and how they each, in their own way, advance the view which St. Thomas proposes. As a corollary, then, St. Thomas is giving us a master-class in anti-Cartesianism. At the same time, St. Thomas’s result is unique:

[A critic, Coulter] twice refers to Thomas’s “conceding that the human form is the only instance of this type of form within the entire order of the universe,” as if it were problematic. It is indeed, but the problem is not a consequence of Thomas’s theory; rather, it is integral to the phenomena of human nature. An attempt to save the problematic phenomena is precisely what leads Thomas to assert different things about the human composite than he does about composites generally.

Ibid., 81–82

6.4. Completing the Negative Inductive Argument (II.56–67)

Recall that St. Thomas begins II.68 with a proposed negative inductive argument. That is, either it is the case that A or B or C … or Z, but it cannot be the case that A, B, C, …, or Y work, therefore it must be the case that Z, the remaining option of the exhaustive division, is true. Previously, I proposed how this disjunction is complete. Note that Aquinas nevertheless states “This can be made evident as follows” (ScG, II.68) That is, a positive case can be made for the remaining option.

Before turning to that case, however, we should review the other disjuncts of his argument and see a major reason why each one is rejected. Recall, from previously, that we saw certain appeals to experience which formed the basis for aspects of these arguments. What about the philosophical reasoning, however, in addition to this?

How not to unify an intellectual substance and a body to get a human being.
Position of …The position and Aquinas’s argument(s) against …
PlatoThey’re united as mover and moved (sailor in a ship) by virtual contact (II.57).
— A mover does not give the mover-mobile system a truly common operation (contrary to experience), or a species as one kind, or being simply (as opposed to accidentally), or a beginning or end of being generated or corrupted (transmigration).
— Note how in ScG, II.58, the unity in substance and in our experience is the basis for many arguments.
AverroesThey’re united in a joint act of thought, separate substance to human bodies (II.59).
— This proposal implies we are not understanders but rather things which are understood by the separate mind; it undermines our self-experience as having understanding, which leads us to infer that there be some principle that is part of our being that accounts for this.
— Note how, in ScG, II.60, the attempted response by Averroes, that we receive our species from the cogitative power, receives about twenty-five separate rebuttals.
AlexanderThe possible intellect results from a particular material mixture, a preparatio (II.62).
— This sort of preparation, since it is only material, fails to explain even vegetative operations, much less sensate or intellectual activities; furthermore, it is more on the side of potency and not act, and thus cannot cause our distinctiveness in kind.
GalenThe soul is a kind of bodily temperament, a complexio (II.63).
— But a complexio is an accidental form, resulting from a substance, and thus cannot explain what something is fundamentally; neither can a complexio account for motion in animals opposed to the nature of mere matter, or for self-rule over the emotions rooted in one’s temperament or complexio.
EmpedoclesThe soul is a kind of harmony or ratio among bodily or material components (II.64).
— The replies are very similar to the replies to Galen.
Other AncientsThe soul is a body, or a variant of sensation, or a sort of imagination (II.65–67)
— A common theme here is to return to the arguments that show why intellectual activity cannot be any one of these: corporeally based, or a kind of sensation, or a sort of imagination, for intellectual activity enjoys universal content.

6.5. The True Unity of the Human Person (II.68)

Now we can turn to St. Thomas’s positive defense of how an intellectual substance can be the form of a body. He provides two conditions (II.68, 3rd):

  1. “One of them is that the form be the principle of substantial being to the thing of which it is the form.”
  2. “… the second condition [is] that the form and matter combine together in one being.”

It does not seem that St. Thomas is asking too much. This is because both conditions are simply part of what makes a substantial form to be such a cause. Condition (1), as Aquinas explains, distinguishes substantial form from the agent or efficient cause of generation. Condition (2) also distinguishes it from an extrinsic agent cause, but this condition also excludes the case of virtual contact.

Aquinas then obviates a certain conceptual roadblock: there is nothing in this notion of form that prevents a subsistent form (intellectual substance) from sharing its existence with matter:

Now an intellectual substance is not hindered by the fact that it is subsistent—as proved above [ch. 51]—from being the formal principle of being to matter, communicating its being to matter, as it were [quasi esse suum communicans materiae]. For it is not unreasonable that the composite and its form itself should subsist in the same being, since the composite exists only by the form, nor does either subsist apart from the other.

ScG, II.68, translation slightly emended from

Note the reason given: Form is the principle of existence of a substance. So, it is not on that account, namely, precisely by being a subsistent form, that an intellectual substance cannot also be the principle of being of a composite.

Still, one might think (as Aquinas must answer arguments to this effect in II.69–70) that there is something intrinsically impossible about a subsistent form sharing its existence such as to be the form of one composite and one existence.  Since it is not possible to argue from conceptual possibility to actuality, St. Thomas proceeds carefully. He must refute the other arguments to negatively defend his claim. He does reply quickly to the concern that this sharing in being or existence seems impossible “because different genera have different modes of being.” This would be a good objection “if this being belonged in the same way to matter as to the intellectual substance. But it is not so.” Rather, the being of the intellectual substance as principle belongs to the material part as recipient and subject.

Now, to positively defend his view, Aquinas can either argue from (a) principles better known to us or (b) from principles better known in themselves. It is the task of the De Anima, the philosophical order of discovery (or, PHIL 3023/S Human Person) to take strategy (a). Since this is the Summa contra Gentiles, and still formally a work of theology making use of metaphysics, St. Thomas chooses strategy (b). (Indeed, doesn’t the latter assume the former?)

It seems to me that St. Thomas makes use of four ideas when employing route (b):

  • “The wondrous connection of things …”
    • This is the Dionysian principle that infimum supremi generis contingere supremum inferioris generis. The human substance provides continuity to the order of kinds.
  • The horizon idea
    • This idea flows from the previous one: “Hence it is that the intellectual soul is said to be on the horizon and confines of things corporeal and incorporeal, inasmuch as it is an incorporeal substance, and yet the form of a body.”
  • The victory of form idea
    • This notes how it is that form can do this by focusing on it as a principle of being, a correlative cause with matter: “the more a form overcomes matter [quanto forma magis vincit materiam], the more one is that which is made from it and matter.”
  • The hierarchy of being
    • This is really in response to an objection: Just because form and matter have one being (Condition (2) above), it does not follow that “matter always equals the being of the form.” 
    • To explain this, Aquinas then sketches out the hierarchy of being (see table below).

Each of these background ideas fleshes out the case for why it can make sense that an intellectual substance is the principle of existence of a body as form. Again, it is important to note that these arguments follow upon strategy (b). It is clearer to us by way of discovery by route (a).

The Hierarchy of Being (in the Physical Cosmos)
Elemental Forms“… these forms are altogether material, and wholly merged in matter.”
Compound Forms“… these, although they do not extend [beyond elemental forms] … nevertheless sometimes produce those effects by a higher power which they receive from the heavenly bodies.” (Contemporary examples: how elements interact via EM or gravity.)
Plant Souls“… we find certain forms whose operations include some which surpass the power of the aforesaid qualities …. They are the principles of movement in living things, which move themselves.”
Animal Souls“Above these forms we find other forms like the higher substances not only in moving, but also in knowing—and thus they are capable of operations to which the aforesaid qualities do not help even organically.
Human Souls“… above all these forms we find a form like the higher substances even as regards the kind of knowledge, which is intelligence: and thus it is capable of an operation which is accomplished without any corporeal organ at all.

Note however Aquinas qualifies this last gradation of forms:

And yet, since the human soul’s act of intelligence needs powers (namely, imagination and sense) which operate through corporeal organs, this by itself shows that the soul is naturally united to the body in order to complete the human species.

It seems to me that this contains an implicitly hierarchical-and-teleological argument for why there is an intellectual substance united to a body. That is, among the genus of intellectual substances, one kind is the lowest, and, in fact, so low that it is incomplete as an intellect such that that it can only achieve its proper act, intellectual activity, with the help of bodily tools (organs, a body). (The Thomistic philosopher and theologian Charles De Koninck makes this argument, at greater length, in his writings.) Note how this teleological reasoning, however, can be held generically in common by the Aristotelian position Aquinas defends as well as by Plato and Averroes. However, the final causality here, why and how the body is for the sake of mind, looks very different when looked at specifically.

6.6. Why St. Thomas Has No Cartesian Interaction Problem (II.69–70)

We are now in the position to follow St. Thomas as he answers prior objections. Recall that this is to make his case negatively that it is possible for an intellectual substance and body to share one existence, to be simply one in being, by their being, respectively, formal and material causes.

Recall that there are still two main series of objections to answer. The first series was originally made in ScG, II.56. The second series of objections was made when describing the Averroist position, in II.59. We will simply note in passing that St. Thomas completes his case by showing, in II.70, how Aristotle cannot be used in support of such views.

Here are the ten objections and their replies, summarized:

The dispute over whether or not an intellectual substance can be the form of a body
Negative arguments in general (II.56)Replies to these (II.69)
1st: One substance in act cannot be made from two substances in act; hence dualism results.1st: The argument presupposes two substances in act prior to their being; but this is not the case.
2nd: Form and matter must belong to one genus, which is not seem possible in this case (intellectual, corporeal belong to different genera).2nd: Form and matter are not two species in one genus, but two principles of one species.
3rd: Then an intellectual form would also be material, which seems impossible.3rd: The intellectual form is not in matter as immersed in matter, but in another way (uniquely, horizon).
4th: What exists in bodily form cannot exist apart from the body, which cannot apply to intellect.4th: Essence vs. power distinction: “According to its essence, it gives being to such and such a body, while according to its power it accomplishes its proper operations.”
5th: If its being were in common with body, then the intellect’s operation would also be bodily, which is not true.5th: The intellectual form is not immersed in matter, but in this activity elevated above it; again, an essence versus power distinction.
Averroist arguments (II.59)Replies to these (II.69)
1st: There is some basis for this conclusion from textual support, authority of Aristotle.1st: The words of Aristotle show only the unmixed character of the principle of the operation.
2nd: Since the intellect “has no intrinsic form” so as to receive other forms intellectually, it cannot be the form of some other thing, e.g., a body.2nd: This conflates form with the principle of intellectual operation, which latter is a power.
3rd: The intellect as form of a body would receive in a bodily way (but then, we wouldn’t know universals).3rd: Aristotle does not intend to exclude that immaterial power from being part of a form.
4th: The intellect as form of a body would receive as prime matter does; but prime matter isn’t a knower.4th: These are not the same kind of reception (intellectual and material).
5th: Intellect is in power infinite (universal to indefinite particulars); thus, cannot be bodily.5th: The power of the intellect not limited, because saying it is part of such a form does not make it a magnitude.

6.7. A Union Both Immediate and Total (ScG, II.71–72)

To say that the intellectual soul is immediately joined to the body is to exclude some other accidents or operations from joining them. The union of body and soul is prior in being. However, note Aquinas’s exception: moving and generation. This exception will raise further questions later on, when we come to the topics of the inception of the human soul and also human death.

To say that the intellectual soul is joined as a whole is to exclude view such as the multiplicity of souls-in-a-soul. The soul is not divided into integral parts throughout the body. Rather, it is an essential whole present in each part of the body: each part of my body is a specifically human part. However, we should note later how potential parts are important (see the last two paragraphs).

6.8. Concluding Questions

The upcoming chapters add nuance and complexity to Aquinas’s account, if one can imagine that. Excerpting from a previous outline, ScG, II.73–89 deals with three topics: the intellectual powers of the human soul in more detail, and then the end of soul-body union as well as its beginning.

To borrow an image from above, St. Thomas’s placement of the human person on the horizon between two realms of being also raises questions about the temporal horizons surrounding an individual human life.

Thomas does not hold that there is individuation of the soul prior to or independent from the body; he does, nonetheless, hold that individuation is not fully explicable in terms of the body. While the induction of other natural forms is part of the process of generation, God infuses the human soul into the human body at an appropriate point in the development of the fetus (II.87). This is yet another indication of how peculiar is the composition of human nature, of the creature that exists on the horizon of the material and the spiritual.

The image of horizon figures prominently in the response to the third objection [in II.80], which is based upon the role of the phantasm in human knowing. Once separate, the soul will not have access to phantasms; it will thus be inoperative and incapable of existing. Thomas’s response is that the separated soul will understand “in a different way,” in the manner appropriate to separate substances: “in its mode of understanding, it will be completely assimilated to the separate substances” . . . . He adds, “Since the human soul exists on the boundary of corporeal and incorporeal substances, on the horizon of eternity and time, it approaches the highest by withdrawing from the lowest” (II.81). The application of the image of horizon in its pristine Platonic sense runs counter to the previous assertion that the soul is naturally the form of the body. Thomas seems here to forget the pertinent Aristotelian comparison of the intellect to the owl in the light of day. As Anton Pegis observes. Thomas creates “a new difficulty in the process of answering an old one.”

Hibbs, Dialectic and Narrative in Aquinas, 94.

That is, the very idea of the close unity of intellectual soul and body which Aquinas had been at such pains to defend now gives rise to philosophical quandaries at the two termini of a human lifespan.

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