THE question of what the universe is made of was still a major concern of Greek philosophers in the fifth century BCE. A native of Akragas in Sicily named Empedocles thought that everything was composed of a single element (known as the Milesian line of thought). Later, however, he took this a step further, identifying four distinct elements – earth, water, air and fire – which in different proportions formed all the different substances in the universe. Developing his ideas from the monism of Parmenides, he argued that these elements must therefore be eternal and unalterable, but reasoned that change was possible if some sort of force altered the mixture of elements.
He suggested that two opposing forces, which he poetically called ‘Love’ and ‘Strife’, caused attraction or separation of the elements and brought about changes in the composition of substances. His classification of the substances later known as the four classical elements was widely accepted by philosophers and was a cornerstone of alchemy until the Renaissance (the transition from the Middle Ages to the Modern Era and covering the span between the 14th and 17th centuries).
Democritus and Leucippus: atomism
A theory of matter proposed by Leucippus and his pupil Democritus was less influential at the time than the ‘four elements’ proposed by their contemporary Empedocles. In retrospect, it seems closer to modern scientific understanding. They suggested that everything in the universe is composed of minute, unalterable and indivisible particles, which they called atoms (from the Greek atomos, uncuttable). These, they argued, are free to move through empty space, combining in constantly changing configurations.
The assertion there is such a thing as a void, an empty space, may be one reason these ideas were originally considered unacceptable. According to their theory, the number of atoms is infinite, and different kinds of atoms with different characteristics determine the properties of the substance they form together. Because the atoms are indestructible, when a substance, or even a human body decays, its atoms are dispersed and reconstituted in another form.