Descartes: Substance, Extension, and the Mechanistic World

Descartes' Concept of Substance

Descartes identified the first element of reality in the self as a thinking substance. He conceived clearly and distinctly that the 'I' did not need other things to exist, thus establishing his concept of substance. The self is real, and what is real is substance. Thinking substance is the primary substance identified, distinct from Aristotle's concept of substance as a substrate for attributes; for Descartes, it is a kind of substance conceived by reason.

Alongside thinking substances, Descartes posits God, identified through the attribute of infinity as the infinite substance. The definition of substance in its purest form belongs to God, for God alone needs no other thing to exist; God exists in Himself and by Himself.

To explain physical phenomena, Descartes reduced them to mathematical and geometrical structures—order and measure. Since geometric figures are measurable through clear orders of points, lines, and surfaces, the essential quality remaining is that which is measurable: extension. He conceived matter stripped of its other qualities, retaining only length, width, and depth, which are the characteristics of extension. Thus, bodies are conceived as extensive substances.

Attributes and Modes in Descartes

Descartes distinguishes between substances, attributes, and modes. Attributes constitute the essence or nature of a substance. The attribute of bodies is extension, the attribute of the self (mind) is thought, and the attribute of God is infinity (from which we know His goodness, eternity, truth, omniscience, omnipotence, etc.). Descartes also defines attributes as always presenting the same form in the substance, essential to its existence and duration.

Modes are the different forms or modifications that attributes can take. For example, extension can manifest as a specific figure or movement. Thought can manifest as imagination, feeling, desire, memory, and so on. However, Descartes suggests all modes of thought can be reduced to two general modes: understanding and will.

Furthermore, one can speak of accidents, but unlike substance, attributes, and modes, they lack objective reality in Descartes' system.

The Mechanistic Paradigm

Descartes advocated for mechanism, leading him to assert that the universe operates like a large machine. All phenomena are explained by the movements of particles within divided matter. Essentially, everything reduces to extension and movement. This view requires discarding the hidden forces of the Renaissance and the final causes central to Aristotelian thought.

Extension's fundamental modes are shape and movement, which Descartes termed primary qualities. Qualities like color, smell, and sound are considered secondary qualities, having only subjective validity.

Like the universe as a whole, plants, animals, and even the human body are viewed as pure mechanisms. To explain their activity, one must not resort to any soul or vital principle, as they obey the same physical laws (forces operating through extension and motion) that govern the rest of the inanimate universe.