Einstein identifies two separate inventions and dates them centuries apart. The first is deductive proof, worked out by Greek philosophers and preserved in Euclidean geometry, which showed that conclusions could be derived with certainty from stated premises. The second is controlled experiment, developed in the Renaissance, which showed that causes could be isolated by deliberate intervention rather than by observation alone.

Neither is sufficient on its own. Deduction can be rigorous without touching the world; experiment can accumulate results without organising them. Modern science, on this account, is what happens when the two are combined.