Atomism

The idea that everything might be made up from large numbers of discrete elements was discussed around perhaps 450 BC by Leucippus and Democritus. Sometime later the Epicureans then suggested that a few types of elements might suffice, and an analogy was made (notably by Lucretius around 100 AD) to the fact that different configurations of letters can make up all the words in a language. But only some schools of Greek philosophy ever supported atomism, and it soon fell out of favor. It was revived in the late 1600s, when corpuscular theories of both light and matter began to be widely discussed. In the early 1800s arguments based on atoms led to success in chemistry, and in the late 1800s statistical mechanics of large assemblies of atoms were used to explain properties of matter (see page 1019). With the rise of quantum theory in the early 1900s it became firmly established that physical systems contain discrete particles. But it was normally assumed that one should think only about explicit particles with realistic mechanical properties—so that abstract idealizations like cellular automata did not arise. (See also pages 1027 and 1043.)