Atomism is the doctrine that the world is made up of small, indivisible (atomos is Greek for uncuttable) elements, goes back to the views of Democritus and Leucippus, and was adopted by Epicurus as reported by Lucretius. This ancient doctrine came to the fore again through the researches of gassendi in the 17th century, and was used by Boyle to explain certain chemical phenomena. It was taken up on a firmer experimental basis by Dalton, and in our own time has become a well-established theory.
Strictly speaking the 'atom' of modern physics is not indivisible (it can be split), but it remains the smallest unit having the property of any stable chemical element. In logic, atomism has inspired the view, favored by some writers, that all complex statements are reducible to atomic statements, logical 'atoms' that cannot be further analyzed.
Atomism is a philosophic doctrine that views indivisible units or entities as the ultimate reality of the physical universe. Usually these entities are conceived as material particles, and the physical universe is seen as embracing all reality.
In ancient Greek philosophy, Leucippus and Democritus, in the 5th century B.C., held that all of reality consists of an infinite number of indivisible, impenetrable, material particles moving in all directions in the void. These atoms are invisible and indestructible, differing from one another only in size and shape. Their random collision produces vortices or whirls, in which, by a sifting action, atoms of like size and shape are brought together. Innumerable worlds, and all things therein, result from such aggregation and combination. Thus all qualitative differences in things are reducible to quantitative difference and mechanical action. Soul consists of the finest and subtlest atoms. Change and variety are explicable as rearrangement of indestructible material particles. In this concept of the universe there is no need for either an ordering intelligence or a divinity.
Following its eclipse in medieval Christian thought, atomism reappeared during the period of early modern science in the mechanistic account of nature given by Pierre Gassendi in the 17th century. It also played a role in the thought of Boyle, Newton, and such Enlightenment thinkers as Diderot and d'Holbach.