Since ancient times, people have searched for explanations of how the world works, and why. Many accept the idea that a divine, external force created and maintains the cosmos, and ancient humans personified the natural world and its behaviors in anthropomorphic deities. In recent centuries, science has developed the tools to answer many of the old questions—why the seasons change, or why the sun rises and sets—with precise, provable answers. As a result, Hawking argues that the need for mythological or religious explanations for the forces in the universe decreases as science improves. By tracing the history of theoretical physics, Hawking explores how new and increasingly more accurate theories and experimentation brought old religious philosophies into question.

Aristotle argued that human reason alone could figure out how the cosmos works. He assumed that all matter was infinitely divisible, that the world was made of five essences, that objects tend to come to rest unless pushed, and that heavy objects fall faster than light ones. Galileo showed, with simple experiments, that heavy and light objects fall at the same rate: he “rolled balls of different weights down a smooth slope,” and “each body increased its speed at the same rate, no matter what its weight” (15).