The break with the ancient world

For roughly two thousand years, the intellectual world of Europe ran on one operating system: Aristotelian-scholastic philosophy. Nature was a purposeful, qualitative cosmos. Things behaved as they did because of their natures and the ends they were striving toward. Knowledge advanced mainly by interpreting authoritative texts — above all Aristotle, absorbed into Christian thought by the medieval Scholastics. “Modern” philosophy is, at its root, the demolition of that system.

The Aristotelian universe was built on four causes, and the one that mattered most for explanation was the final cause (causa finalis) — the purpose or goal of a thing. Ask why a rock falls, and the answer was teleological: earth is heavy, its natural place is the centre of the cosmos, so it strives downward to where it belongs. Fire is light, so it strives up. The heavens, being perfect, move in perfect circles. Nature was shot through with goals, tendencies, and value.

What broke the old system was the Scientific Revolution. Copernicus (De revolutionibus, 1543) moved the Earth out of the centre and set it spinning around the sun. Kepler replaced the sacred circles with ellipses and captured planetary motion in exact mathematical laws. Galileo turned his telescope on the heavens — mountains on the moon, moons around Jupiter, phases of Venus — and found a physics of uniform matter obeying the same rules everywhere, on Earth and in the sky alike.

The result was a new picture of nature: mechanical and mathematical. The world is matter in motion — particles of stuff pushing on one another, describable by size, shape, position, and speed. Final causes are expelled: a planet does not orbit because it seeks perfection but because of quantitative laws of motion. This is the mechanical philosophy, and it would reach its triumph in Newton’s Principia (1687), where a single law of gravitation governs both a falling apple and the moon.

Here is why the scientists’ revolution became a philosophical crisis. If the authoritative picture inherited from Aristotle and the Church could be so completely wrong — about the very structure of the cosmos — then how much of what we take ourselves to know is similarly rotten? Authority and tradition had failed as guarantees of truth. The educated world faced a frightening question: what, if anything, can we be certain of?

The response that defines the age is the demand for secure foundations. Rather than patch the old edifice, the leading thinkers proposed to tear it down and rebuild knowledge from the ground up, on foundations so solid that no future revolution could topple them. Francis Bacon wanted a fresh start in method for the sciences; Descartes wanted a fresh start for knowledge itself. The metaphor of the age is construction: raze the shaky building, dig to bedrock, build again.

This produces the great reorientation of philosophy. Ancient and medieval thinkers led with metaphysics — questions about what the world is made of and what is ultimately real. The moderns lead instead with epistemology — questions about how, and how far, we can know anything at all. Before you can say what reality is, you must first secure your right to make any claim about it. The theory of knowledge moves to the front of the queue.