Mind, Matter, and Meaning

Thomas Aquinas

Thomas Aquinas (c. 1225–1274) was a Dominican friar, philosopher, and theologian whose writings bring Aristotelian philosophy into sustained discussion with Christian theology. His account of human nature combines bodily life and sensory cognition with an argument that intellectual activity has an immaterial principle. It therefore resists classification as either modern physicalism or Cartesian substance dualism.

Major Works

  • Summa Theologiae — a theological synthesis organized through questions, objections, and replies. Part I, questions 75–89, examines the human soul, its powers, and cognition.
  • Disputed Questions on the Soul (Quaestiones disputatae de anima) — arguments about the soul’s nature and relation to the body.
  • Commentary on Aristotle’s De Anima — interpretation of Aristotle’s account of life, perception, and intellect.

Soul and Body

Following Aristotle, Aquinas treats the soul as the substantial form of the living body: the principle in virtue of which the body is one living organism with its characteristic powers. “Form” here means more than shape, and the soul means more than conscious thought. Nutrition, sensation, and movement are among the activities of living beings that the doctrine explains.

The human being is a body–soul composite, not a soul using a body as an external instrument. Aquinas argues that sensation belongs to the living composite and requires bodily organs. He nonetheless holds that the rational soul can subsist apart from the body. These claims are intended to be compatible: a subsisting soul remains the form of a body, and its separated condition does not constitute complete human life.

Perception and Intentional Reception

Aquinas distinguishes natural reception of a form from its cognitive or intentional reception. A colored thing can affect a sense organ without making the organ colored in the same way. Cognition receives a likeness or species through which the object becomes known; the knower need not become a material duplicate of the thing known.

This account belongs to the medieval background of theories of representation and intentionality. It should not be identified without qualification with a modern theory of neural vehicles and semantic content. Aquinas’s explanation uses Aristotelian form, powers, and actuality, and different medieval thinkers developed different accounts of cognitive species.

Abstraction and Intellectual Knowledge

Human understanding ordinarily begins from sensory experience. The internal senses provide phantasms, sensory images from which the active intellect abstracts intelligible species. Through these the possible intellect comes to understand universal natures. Aquinas does not treat abstraction as extracting a physically detached object from an image; it makes an intelligible aspect available to intellectual cognition.

In Summa Theologiae I, question 85, article 2, he distinguishes what the intellect understands from that by which it understands. The intelligible species normally functions as a means of knowing the object, not as a substitute object that alone is known. Species can themselves become objects of reflection, but that is a further act. This distinction is central to discussions of whether his account is a direct or representational realism.

The Argument for a Subsisting Intellect

Aquinas argues that a power capable of knowing bodily natures universally cannot itself have the determinate bodily nature that would restrict its reception of them. He therefore holds that intellectual operation does not occur through a bodily organ in the way seeing does. From an operation belonging to the soul itself, he argues that the rational soul subsists; he also defends its incorruptibility.

This is an argument about the nature of intellectual cognition, not merely an appeal to revealed theology. Its premises about universality, bodily determination, and the dependence of operation on being are disputed. A contemporary critic may question whether universal thought requires an immaterial capacity at all. A Thomist replies that dependence on sensory and neural conditions does not by itself identify the intellectual act with an organ’s operation; the philosophical issue concerns what that act can grasp.

Interpretation and Continuing Disputes

Two difficulties organize later discussion. First, how can the same soul function as the form of a body and subsist when separated from it? Aquinas’s answer distinguishes complete human nature from the soul’s capacity for independent existence, but interpreters disagree over the resulting account of personal identity. Second, do species explain knowledge of the external world or interpose representations between knower and object? The “by which” distinction supports a realist reading, while further questions remain about likeness, truth, and how cognitive forms relate to their objects.

Aquinas’s body-dependent account of ordinary human thought also prevents the separability argument from being read as a claim that embodied life is dispensable. His psychology combines the rational soul’s subsistence with the role of sensation and imagination in human understanding. Later Thomist interpretations emphasize different parts of that combination.

  • Aristotle — form, matter, and the powers of living things.
  • Brentano — a later recovery of Scholastic intentionality terminology.
  • Intentionality — cognition directed toward its objects.
  • Embodiment — bodily conditions of sensation and thought.
  • Dualism — comparisons between hylomorphism and separable-mind theories.

Further Reading

  • Aquinas, Thomas. Summa Theologiae, Part I, questions 75–89; especially question 75, articles 2 and 4, and question 85, articles 1 and 2.
  • Aquinas, Thomas. Disputed Questions on the Soul.
  • Aquinas, Thomas. Commentary on Aristotle’s De Anima.
  • Lagerlund, Henrik. “Mental Representation in Medieval Philosophy.” Stanford Encyclopedia of Philosophy.
  • Mind, Matter, and Meaning — Gordon Swobe’s book-length argument about mind, consciousness, and meaning; an author position, not an endorsement by this reference page or its subjects.