A philosophical doctrine at least as old as Democritus, and plausibly viewed as an attempt to combine an a priori conviction of the unchangeable and immutable nature of the world with the variety and change of things as we know them. This is the conviction that to understand complexity and change at one level it is necessary to find underlying unity at another level. In early Greek atomism quantitative change arises from the shifting configurations and quantities of atoms, which are themselves eternal, impenetrable, identical in [?] nature, and unchanging. After Aristotle, atoms were allowed to be subject to change: what is unchanging was not necessarily corpuscular in nature.
The revival of atomism in the 17th century owed more to the rise of empirical science. Descartes produced the first serious departure from Democritus and Aristotle, identifying matter and extension, but differentiating corpuscles only in terms subject to mechanical and mathematical treatment (velocity, mass). Leibniz was the most persistent critic of 17th-century atomism. Atoms offend against the principle of sufficient reason, for there could be no reason why a particular atom occupies a particular position rather than any other. But on more physical grounds Leibniz held that they involve discontinuities in nature (density changes discontinuously at their boundaries); that their own cohesion would require a perpetual miracle; and that no theory of their inelastic collisions is tenable. His arguments were revived and turned into a positive field theory by Boscovich in the following century. Leibniz also held that whatever had extension was divisible, so that true atoms, the indivisible foundation stones, become quasi-mental things, with some of the qualities of the soul.
Further developments of atomism were hindered by the absence of a chemistry that could find a workable criterion of the difference between elements and compounds, and this was first delivered by the chemists Lavoisier (1743–94) and Dalton (1766–1844). The complexity of atoms in the chemical sense is now attested by the proliferation of subatomic particles. What remains of philosophical and methodological interest is the extent to which each level of complexity is to be understood by postulating some identical and unchanging constituents at a lower level, as Democritus originally supposed.
http://www.ellopos.net/elpenor/greek-texts/ancient-greece/history-of-philosophy/leucippus-democritus.asp An introduction to ancient Greek atomism
http://faculty.washington.edu/smcohen/320/atomism.htm A summary of the debates over Democritus’s atomism