Descartes' Dualism: Mind, Body, and the Pineal Gland

Descartes' Dualism: The Division of Substance

The reality, according to Descartes, is divided into three classes of things (res) or substances:

• Res Infinita (Infinite Substance): God, the creator of all things, who ultimately serves as the guarantor of our cognitive ability.
• Res Cogitans (Thinking Substance): The ego or soul, representing the innermost nature of man. The ego, as the subject of all intellectual activity, is composed of two faculties: the intellect and the will.
• Res Extensa (Extended Substance): External bodies or material substances.

The Cartesian View of Human Nature

The separation between spiritual substance (Res Cogitans) and extended substance (Res Extensa) profoundly affects the Cartesian view of human beings. The human body, like all other bodies, is subject to natural laws, mechanics, and determinism. In contrast, the soul is free and possesses its own initiative and spontaneity.

The Problem of Interaction: Body and Soul

Descartes, following the Christian tradition, asserts that man is composed of both body and soul. However, the relationship between these two distinct substances poses significant difficulties. In this respect, Article 34 of The Passions of the Soul states that "the soul has its headquarters in the small gland found in the middle of the brain," referring to the pineal gland. From this point, the soul supposedly controls moods, nerves, and even blood.

A major philosophical hurdle arises because the soul, being unextended, cannot physically move the body. To move material, contact is required; what moves and what is moved must be contiguous in space. This implies that both interacting substances should be extended.

The Pineal Gland Analogy

Despite the logical difficulties, Descartes insists on the soul's location in the brain, comparing it to a plumber in a large water distribution center who can allow water to drain through one pipe or another. Similarly, the soul in the brain would control the passage of the "humors" or "heat" of the blood through various cavities, regulating intensity, etc. The resulting motion of the body is determined by how these hot fluids pass through specific vessels.

Legacy of the Mind-Body Problem

As demonstrated, Descartes' solution regarding the pineal gland does not appear entirely consistent. Consequently, the fundamental problem of communication between the body and the soul was inherited by his philosophical successors.