The fundamental questions of what the world is made of, and where matter came from, are some of the oldest. In the 6th century BCE, Greek philosophers such as Thales and Anaximenes proposed that all substances were modiﬁcations of more intrinsic substances, the main candidates being water, air, earth, and ﬁre. In the 5th century BCE, Empedocles claimed that everything was a mixture of all four of these substances, or elements. His near-contemporary Democritus developed the idea that the universe is made of an inﬁnite number of indivisible particles called atoms. Finally, in the 4th century BCE the influential scholar Aristotle added a ﬁfth element, ether, to Empedocles’four. Although Aristotle was skeptical of the idea of atoms, it is remarkable that the concepts of both atoms and elements had been proposed more than 2,000 years before either was proved to exist.