Metaphysics: The Nature of Reality

René Descartes (1596–1650) inaugurated modern philosophy with a radical methodological experiment: he decided to doubt everything he could possibly doubt, and to accept only what survived this skeptical scrutiny. His goal was to find an indubitable foundation for knowledge — something so certain that not even an all-powerful deceiving demon could make it false.

The process of systematic doubt swept away a surprising amount. Could he doubt the evidence of his senses? Yes — his senses had misled him before, and the arguments for dreaming and demonic deception were available. Could he doubt the truths of mathematics? Even these might be deceptions planted by an evil demon. What survived?

The famous answer: "I think, therefore I am" (cogito ergo sum). The very act of doubting is a form of thinking; and if I am thinking, I must exist as a thinking thing. Even the most radical skeptic cannot coherently deny her own existence as a thinking being — because the denial is itself a thought. This is Descartes' indubitable foundation: the existence of the thinking self.

From this starting point, Descartes argued for the existence of God (a perfect being cannot fail to exist), and through God's goodness, for the general reliability of our rational faculties. The argument is controversial at every step, but the method — radical doubt as a tool for finding certainty — transformed philosophy.

Having established the self as a thinking thing, Descartes faced an acute problem: what is the relationship between this thinking self (the mind) and the body? He concluded that they are radically different substances: the mind is unextended, non-spatial, purely thinking; the body is extended, spatial, purely mechanical. This position is called Cartesian dualism, and it generates a problem that has not been resolved in 400 years.

The problem: if mind and body are entirely different substances, how do they interact? When I decide to raise my arm, a mental event (my decision) seems to cause a physical event (my arm rising). But how can something non-spatial and unextended cause spatial, extended matter to move? Descartes' answer — that they interact through the pineal gland — was unsatisfying even to his contemporaries.

The alternatives to dualism each carry their own difficulties. Physicalism (or materialism) holds that only physical things exist and that mind is either identical with, or reducible to, or realized in, brain states. This is now the dominant position in philosophy of mind, but it faces the "hard problem of consciousness": it is unclear how and why physical processes give rise to subjective experience — the felt quality of seeing red or feeling pain.

David Hume (1711–1776) subjected the concept of the self to radical empiricist scrutiny and concluded that it might not exist in the way we think it does. His argument: if you look for a "self" in experience, what do you actually find? You find a stream of perceptions — thoughts, feelings, sensations, memories — but no separate "self" underlying them, the way a peg underlies the hats hanging on it. What we call the "self" is just a "bundle" of perceptions, not a substantial entity that has them.

This bundle theory has striking implications. If there is no substantial self, then personal identity over time becomes puzzling. What makes you the same person you were ten years ago? Not a persistent soul or substance, since Hume denies these exist. Perhaps just psychological continuity — a chain of connected memories and experiences. But chains can break, and memories can be false. The problem of personal identity — first sharply posed by Locke, radicalized by Hume — remains genuinely open.