Atomism is a Pre-Socratic school of thought from ancient Greece, established in the late 5th Century B.C. by Leucippus of Miletus (5th Century B.C.) and his more famous student, Democritus. It teaches that the hidden substance in all physical objects consists of different arrangements of atoms and void (see the section on the doctrine of Atomism for more details).
No writings by the movement's founder, Leucippus, have survived, and we have just a few fragments of the writings of Democritus in secondhand reports, sometimes unreliable or conflicting. Much of the best evidence is that reported by Aristotle in his criticisms of Atomism, which he regarded as an important rival current in natural philosophy.
Epicurus studied Atomism with Nausiphanes (c. 325 B.C.) who had been a student of Democritus. Although Epicurus was certain of the existence of atoms and the void, he was less sure he could adequately explain specific natural phenomena such as earthquakes, lightning, comets, or the phases of the Moon. He went on to found his own school of Epicureanism.
Of Democritus' and Epicurus' followers, perhaps the most notable was the Roman poet and philosopher Lucretius (c. 99 - 55 B.C.) whose "On the Nature of Things" was one of the definitive works of Epicureanism, but also of Atomism. It argues that the universe and all substance is eternal, composed of atoms moving in an infinite void and nothing else, and that the human soul also consists of minute atoms that dissipate into smoke when a person dies. It depicts Epicurus as the hero who crushes the monster Religion through educating people about what is possible and what is not possible in a world composed of atoms.
Aristotelianism eclipsed the importance of the Atomists, and there was little interest expressed in the idea throughout the whole of the medieval period until its resurrection in the 16th and 17th Century, although the Islamic Ash'arite school of philosophy, notably al-Ghazali (1058 - 1111), propounded a type of hybrid Atomism where atoms are the only perpetual, material things in existence, and all else in the world is “accidental” (lasting for only an instant), and contingent events are the direct result of God’s constant intervention.
Much of the renewed interest in Atomism in the 16th and 17th Century was precipitated by scientific advances, particularly those of Nicolaus Copernicus (1473 - 1543) and Galileo Galilei (1564 - 1642), who himself converted to Atomism when he found that his corpuscular theory of matter and his experiments with falling bodies and inclined planes contradicted the mainstream Aristotelian theories. The English philosophers Sir Francis Bacon and Thomas Hobbes were both confirmed Atomists for a time, as was Giordano Bruno (1548 - 1600) in Italy.
However, the main figures in the rebirth of Atomism were the French philosophers René Descartes and Pierre Gassendi (1592 - 1655), and the Irish philosopher and scientist Robert Boyle (1627 - 1691).
Descartes’ mechanical philosophy of corpuscularism (that everything physical in the universe is made of tiny “corpuscles” of matter, and that sensations, such as taste or temperature, are caused by the shape and size of tiny pieces of matter) had much in common with Atomism, and may be considered in some sense another version of it, although for Descartes there could be no void, and all matter was constantly swirling to prevent a void as corpuscles moved through other matter. Descartes was also firm on the concept of mind/body duality, which allowed for an independent realm of existence for thought, soul and, most importantly, God.
Pierre Gassendi was a French priest and natural philosopher, who set out to “purify” Atomism from its heretical and atheistic philosophical conclusions. He formulated his atomistic conception of mechanical philosophy partly in response to Descartes, particularly opposing Descartes’ reductionist view that only purely mechanical explanations of physics are valid.
Robert Boyle's form of Atomism, which came to be accepted by most English scientists, was essentially an amalgamation of the two French systems. He arrived at it after encountering problems reconciling Aristotelian physics with his chemistry experimentation.
Roger Boscovich (1711 - 1787) provided the first general mathematical theory of Atomism, utilizing principles of Newtonian mechanics. Then, in the early 19th Century, John Dalton (1766 - 1844) developed his atomic theory in which he first proposed that each chemical element is composed of atoms of a single, unique type, which can combine to form more complex structures (chemical compounds).
Although philosophical Atomism led to the development of early scientific atomic theory, modern science has shown that atoms in the chemical sense are actually composed of smaller particles (electrons, neutrons and protons), and that these in turn are composed of even more fundamental particles called quarks. Although the principle can still theoretically apply, there are few, if any, modern-day atomists.
The Atomist idea that anything might ultimately consist of an aggregation of small, indivisible units later lent itself to other fields, such as Social Atomism (the sociological view that society is composed of individuals rather than social institutions) and Bertrand Russell's Logical Atomism (an attempt to identify the atoms of thought, the pieces of thought that cannot be divided into smaller pieces of thought).