Hylomorphism

Aristotle applies his theory of hylomorphism to living things. He defines a soul as that which makes a living thing alive.[15] Life is a property of living things, just as knowledge and health are.[16] Therefore, a soul is a form—that is, a specifying principle or cause—of a living thing.[17] Furthermore, Aristotle says that a soul is related to its body as form to matter.[18]

Hence, Aristotle argues, there is no problem in explaining the unity of body and soul, just as there is no problem in explaining the unity of wax and its shape.[19] Just as a wax object consists of wax with a certain shape, so a living organism consists of a body with the property of life, which is its soul. On the basis of his hylomorphic theory, Aristotle rejects the Pythagorean doctrine of metempsychosis, ridiculing the notion that just any soul could inhabit just any body.[20]

According to Timothy Robinson, it is unclear whether Aristotle identifies the soul with the body's structure.[21] According to one interpretation of Aristotle, a properly organized body is already alive simply by virtue of its structure.[22] However, according to another interpretation, the property of life—that is, the soul—is something in addition to the body's structure. Robinson uses the analogy of a car to explain this second interpretation. A running car is running not only because of its structure but also because of the activity in its engine.[23] Likewise, according to this second interpretation, a living body is alive not only because of its structure but also because of an additional property: the soul, which a properly organized body needs in order to be alive.[24] John Vella uses Frankenstein's monster to illustrate the second interpretation:[25] the corpse lying on Frankenstein's table is already a fully organized human body, but it is not yet alive; when Frankenstein activates his machine, the corpse gains a new property, the property of life, which Aristotle would call the soul.

Some scholars have pointed out a problem facing Aristotle's theory of soul-body hylomorphism.[26] They argue that a living thing's matter is its body and the body needs a soul in order to be alive. Similarly, a bronze sphere's matter is bronze, which needs roundness in order to be a sphere. Now, bronze remains the same bronze after ceasing to be a sphere. Therefore, it seems that a body should remain the same body after death.[27]

Bronze however is not alive and changing its shape does not involve a substantial change. Moreover, Aristotle explicitly denies that a body remains a body after death.[28] A corpse is only equivocally a body because Aristotle states that a body that has lost its soul is no longer potentially alive.[29] To lose the potentiality of life is to be a corpse. Living bodies are thus a composite of soul and matter: the body lives insofar as its soul is actualizing its matter. The body and soul are not two distinct things but as one substance. That is why Aristotle defines the body as having life potentially and the substantial form as the potential body's life source. Aristotelians need to take care not to use the word "body" equivocally to refer to both living things and corpses.

The soul as the body's substantial form enables personal identity to persist over time. Consider the fact that a living body is constantly replacing old matter with new.[30] A five-year-old body consists of different matter than does the same person's seventy-year-old body. If the five-year-old body and the seventy-year-old body consist of different matter, then what makes them the same body? The answer is that the same soul has been animating matter into that person's body.Because the five-year-old and the seventy-year-old bodies share a soul—that is, the person's life—we can identify them both as the body. Apart from the soul, we cannot identify what collection of matter is the body. Therefore, a person's body is no longer that person's body after it dies.

Aristotle says that the intellect (nous), the ability to think, has no bodily organ (in contrast with other psychological abilities, such as sense-perception and imagination).[31] Aristotle distinguishes between two types of intellect.[32] These are traditionally called the "passive intellect" and the "active (or agent) intellect".[33] The passive intellect is like clay; it can become anything and is subject to change. The function of the active intellect is to use forms abstracted from physical things to activate the passive intellect to acts of understanding. Passive intellect is also the storehouse of understood ideas. Aristotle says that the "active (or agent) intellect" is not mixed with the body[34] and suggests that it can exist apart from it.[35]

Aristotle's texts on the agent intellect have given rise to diverse interpretations. Some following Averroes (Ibn Rusd 1126-1198) argue that Aristotle equated the active intellect with a divine being who infuses concepts into the passive intellect to aid human understanding. Others following Aquinas (1225-74) argue that the Neo-platonic interpretation is a mistake: the active intellect is actually part of the human soul. The controversy matters insofar as the Neo-platonic interpretation of Aristotle holds that he rejected personal immortality, while the Thomistic interpretation holds that Aristotle used the agent intellect to establish personal immortality. Others interpret Aristotle as arguing that a person's ability to think (unlike his other psychological abilities) belongs to some incorporeal organ distinct from his body.[36] This would amount to a form of dualism.[37] However, according to some scholars, it would not be a full-fledged Cartesian dualism.[38] This interpretation creates what Robert Pasnau has called the "mind-soul problem": if the intellect belongs to an entity distinct from the body, and the soul is the form of the body, then how is the intellect part of the soul?[39] Yet, another interpretation distinguishes the passive intellect and the agent intellect and considers the passive intellect to be a property of the body, while the agent intellect is a substance distinct from the body.[40][41] Some proponents of this interpretation think that each person has his own agent intellect, which presumably separates from the body at death.[42][43] Others interpret the agent intellect as a single divine being, perhaps the unmoved mover, Aristotle's God.[44][45] Still others[46] argue that Aristotle held that an individual form is capable of having properties of its own.[47] According to this interpretation, the soul is a property of the body, but the ability to think is a property of the soul itself, not of the body. If that is the case, then the soul is the body's form and yet thinking need not involve any bodily organ.[48]

Many medieval theologians and philosophers followed Aristotle in seeing a living being's soul as that being's form—specifically, its substantial form. However, they disagreed about whether X's soul is X's only substantial form. Some medieval thinkers argued that X's soul is X's only substantial form animating the entire body of X.[50] In contrast, other medieval thinkers argued that a living being contains at least two substantial forms—(1) the shape and structure of its body, and (2) its soul, which makes its body alive.[51]

Thomas Aquinas claimed that X's soul was X's only substantial form, although X also had numerous accidental forms that accounted for X's nonessential features.[52][53] Aquinas defined a substantial form as that which makes X's matter constitute X, which in the case of a human being is also able to transcend the limitations of matter and establish both the rational capacity[54] and natural immortality of human beings. Nevertheless, Aquinas did not claim that human persons were their disembodied souls because the human soul is essentially a substantial form activating matter into the body. He held that a proper human being is a composite of the rational soul and matter (both prime matter and individualized matter).[55][56] So a soul separated from its body does not become an angel but retains its orientation to animate matter, while a corpse from which the soul has departed is not actually or potentially a human being.[57]

Eleonore Stump describes Aquinas' theory of the soul in terms of "configuration". The body is matter that is "configured", i.e. structured, while the soul is a "configured configurer". In other words, the soul is itself a configured thing, but it also configures the body.[58] A dead body is merely matter that was once configured by the soul. It does not possess the configuring capacity of a human being.

Aquinas believed that rational capacity was a property of the soul alone, not of any bodily organ.[59] However, he did believe that the brain had some basic cognitive function.[60] Aquinas’ attribution of rational capacity to the immaterial soul allowed him to claim that disembodied souls could retain their rational capacity as his identification of the soul's individual act of existence allowed him to claim that personal immortality is natural for human beings. Aquinas was also adamant that disembodied souls were in an unnatural state[61] and that the perfection of heaven includes God miraculously enabling the soul to function once again as a substantial form by reanimating matter into a living body as promised by the doctrine of the resurrection of the dead.