Aquinas, Plato, and Aristotle on the Soul's Nature

Introduction: Greek and Christian Foundations of the Soul

The Greeks believed that the soul is an essential part of the body since it gives it life. The soul thinks, feels, and chooses. The interaction between body and soul influences each other, giving rise to the concept of dualism. They also viewed the soul as a simple form without any parts. Plato postulated that the soul is separated from the body, and while the body degenerates, the soul departs to form another life at death. Contrastingly, Aristotle considered the soul as a "form" that cannot exist without the body. The following study explicates Aquinas's views of the soul whilst appreciating the contributions of Plato and Aristotle on the topic.

Christian theology subscribes to the concept of life after death, and it was therefore natural to adopt Plato's idea of the soul. However, St. Thomas Aquinas was keen on developing a conception of the soul based on Aristotle's concept while accommodating Christian doctrine. According to Aquinas, the soul was indeed a form, but a special one that could exist briefly without being embodied. In his postulation, human beings are made of body and soul, matter and form, and death entails the separation of the body from the soul. Therefore, the human body ceases to exist while the human soul survives and continues to exist after death. Aquinas described the human soul as the substantial form — unique among the forms of material substances — because it can survive in separation from the body, as it does after death and before resurrection.

Modern views on the soul hold that human beings have souls as an essential part of the body, yet the soul can also be separated from it, as depicted by Aquinas and Plato. However, Aristotelianism argues that humans do not have souls in that sense, and are instead complex beings possessing properties absent in inanimate things. Conceptualizing the soul as something separable from the body therefore implies that it ceases to exist at death once the body stops functioning — a conclusion Aquinas sought to resist.

The Nature and Existence of the Soul

The form of something is expressed in its pattern of reaction, appearance, and shape. According to Aristotle, the form of a living thing is its soul. While animals have sensitive souls, plants have vegetative souls. Humans have intellectual or rational souls that determine how they think and behave. For Aristotle, forms are universal — meaning the same form can be instantiated in multiple dissimilar things. For instance, the form of a table can be instantiated in various tables, and what differentiates one from another is the matter used in making it. Aristotle asserts that matter makes up human beings, and that no form can exist when separated from the particular substance in which it is represented. Therefore, it is impossible to separate a human soul from its body. Aristotle believes that the soul has parts, and that only some of them — such as those responsible for thinking — may be separable.

In contrast, Aquinas asserts that no bodily form is necessary to facilitate thinking. As such, the soul can continue to exist as a whole without a body. This claim aligns with the typical notion of life and death represented in Christian doctrine. He maintains that the soul continues to exist after the body is destroyed, then reunites with it during resurrection. Aquinas clarifies that the soul survives during the period between death and resurrection. Therefore, the soul is an essential part of human beings whose nature is to unite with the body and to function properly within it, yet it is capable of a limited independent existence after death.

Unlike Aristotle, who considers form as universal and the human form as capable of being instantiated in various people, Aquinas maintains that each form is inclined toward a certain body — particularly the one from which it was initially separated at death. Duns Scotus considered this explanation insufficient, arguing that an inclination cannot be bare and must be grounded in something actually existing within the soul. He drew an analogy: some substances incline toward liquefaction at a particular temperature only because of their chemical constitution. Thus, Scotus maintains that souls are individual forms and not mere universals. Claiming that souls can survive in separation from the body must therefore be substantiated by specifying what constitutes this soul and what connects it to the original person.

Plato maintains that it is difficult to analyze what makes a given chunk of matter the same as a prior one. In his view, Socrates is indistinguishable from Socrates' soul — unlike Plato, who treats them as separate. Essentially, Socrates is not his body but his soul. The soul's relationship to the body is analogous to a captain's relationship to a ship: though the captain governs the ship, the captain remains a separate and complete individual who exists independently of it. Souls, in this view, originate from a higher realm and enter the material body to animate it. Plato maintains that if souls live upright lives and achieve total purification, they will ultimately be reintegrated into their primitive spiritual origin. His understanding of the soul is primarily situated in a religious and spiritual context. He provides four attributes of the soul: it is the principle of life; it is immaterial; the rational soul is eternal and immortal; and the soul and body are united in an accidental rather than a natural manner.

Based on Aquinas's view, all that has happened to a soul in the past, when it was combined with a particular body, and all that may happen to it in the future does not constitute what makes it a soul in the present. It must, therefore, have something internal now. Religious devotees who believe that human beings can exist without their bodies, even briefly, must uphold this principle. Similarly, any believer who holds that there is life after death — even if souls cannot exist without embodiment — must maintain a coherent account of that continuity.

Aristotle's arguments regarding post-mortem existence maintain that life ends after death. Although Aquinas adopted several concepts from Aristotle in his broader work, he could not employ Aristotle's philosophy when exploring the concept of life after death. Noticing that there is no Aristotelian text offering a clear solution for the immortality of the mind and soul, Aquinas turned to Platonism. He acknowledged the resistance he encountered when using Aristotelian philosophy to determine the status of the soul after death, and his intellectual honesty led him to adopt Plato's solution where Aristotle's fell short. Aristotle, in particular, does not adequately explain how the soul could remain capable of intelligent activity without being embodied.

Aquinas and Plato agree that while the soul can function intelligently whether united with the body or not, its natural state is to remain embodied, because the relationship between soul and body is not accidental but essential. Plato first introduced the idea that human existence and the soul lie at the interface between the temporal physical world and the non-physical domain. Christian philosophers describe this as a split-level existence in which humans belong to the invisible world through their souls and to the visible world through their bodies. Human beings thus serve as the link between both worlds and stand at the summit of the visible world as animals capable of reasoning.

Corruptionists and survivalists hold conflicting opinions regarding Aquinas's views of the soul and the status of individuals during the interim period between death and resurrection. According to corruptionists, an individual ceases to exist after death and returns to existence at the resurrection. Survivalists, by contrast, contend that the soul persists during the interim period between death and resurrection. Further questions arise concerning the identity of the resurrected body and the extent to which it resembles the original. For instance: do the atoms of the original body reconstruct into a bodily form? What happens when those atoms cease to exist? What percentage of original atoms is sufficient to make the body the same? These questions define the core concerns regarding personal identity after resurrection. The conclusion suggested is that when atoms combine to form a human body at resurrection, the result may not be a perfect replica of the original body. It therefore seems safe to conclude that the degree of human survival is defined not by the body alone but by the soul.

Substance dualism holds that the body is a distinct substance from the soul, the latter being essential to human identity. Critics of this position find it incoherent to suppose that humans could survive without the body, since there is no clear mechanism explaining how the loss of the body could be consistent with the continued capacity to think. Other philosophers challenge substance dualism's account of causal interaction between soul and body, arguing that it fails to explain how soul states cause brain states and vice versa.

Aristotle opposes Plato's dualism, believing that the soul and body combine to form a single substance. He considers dualism problematic: if the soul is fundamentally different from the body, the need for their unity becomes difficult to explain. For both Aquinas and Aristotle, it is necessary that the body unite with the soul, and the body is essential for the soul to exercise its vital capacities. They maintain that the soul requires the body because sensation — including acts of seeing — requires bodily organs, and intellectual activity in turn requires sensation, which occurs only in the presence of the body. To this end, human beings need a body to perform their proper functions. Aquinas also disagrees with Plato's claim that the soul or mind alone constitutes a human being. He maintains that a human being cannot be a mere mind without a body. Accordingly, the human soul remains the essential form of the body, and together they form one substance.

Conclusion: Aquinas's Synthesis of Plato and Aristotle

Aquinas adopts ideas from both Plato and Aristotle when explaining the concept of the soul. He employs Platonism because its framework offers a logically coherent account of life after death — an outcome that Aristotle's philosophy could not provide. Aquinas believed in the harmony between faith and reason, and he was compelled to clarify the concept of the afterlife, which had remained highly speculative. He acknowledged the limitations of philosophical ideas and recognized that they do not capture reality conclusively. Believing that knowledge extends beyond purely philosophical constraints, he applied Platonic features as a significant supplement to his Aristotelianism when exploring fundamental questions about the soul.

While Plato and Aristotle differ in their explanations of the soul, death, and resurrection, they share common ground in their broader philosophical and religious contexts. Aquinas therefore adopts Plato's concepts where they complement the limitations found in Aristotle's thought, producing a synthetic account of the soul and the afterlife that remained consistent with Christian doctrine. In this way, Aquinas's treatment of the soul stands as a landmark example of philosophical synthesis: drawing on the best of the ancient tradition to address questions that neither Plato nor Aristotle could answer alone.

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