Descartes

René Descartes, a Jesuit-educated Frenchman who soldiered briefly and settled in the Dutch Republic for twenty years of quiet work, made doubt itself the foundation of certainty and inaugurated modern philosophy.

The Meditations on First Philosophy (1641) stages the experiment: suspend everything that can possibly be doubted — the senses, the body, mathematics, even the world, imagining a deceiving demon of utmost power. What survives is the doubter: cogito, ergo sum, I think, therefore I am. On that single indubitable point he rebuilds: clear and distinct ideas are trustworthy, he argues, because a perfect God exists — proven from the very idea of perfection in an imperfect mind, and from the ontological argument revived in the Fifth Meditation — and a perfect God is no deceiver.

Critics from his own day forward noted the circle: clear ideas certify God, God certifies clear ideas. Yet the architecture stood. Mind and body emerged as two distinct substances — thinking thing and extended thing — interacting mysteriously (he guessed at the pineal gland), and the "mind-body problem" that still organizes philosophy of mind and neuroscience-of-religion debates is his bequest.

He was cautious with the Church — he shelved his Copernican treatise The World when Galileo was condemned — yet his method carried a quiet radicalism: truth begins not from scripture or Aristotle but from the solitary reasoning self, with God entering as a conclusion rather than a premise. The Discourse on Method (1637), written in French for any intelligent reader, made the revolution public.

He died in Stockholm in 1650, tutoring Queen Christina at five in the morning in a Swedish winter. Within a generation, every European philosophy — and every theology — had to begin where he began: inside the doubting mind.