  The work of Democritus has survived only in secondhand reports, sometimes unreliable or conflicting.  Much of the best evidence is that reported by Aristotle, who regarded him as an important rival in natural philosophy.  Aristotle wrote a monograph on Democritus, of which only a few passages quoted in other sources have survived. Democritus seems to have taken over and systematized the views of Leucippus, of whom little is known.  Although it is possible to distinguish some contributions as those of Leucippus, the overwhelming majority of reports refer either to both figures, or to Democritus alone; the developed atomist system is often regarded as essentially Democritus'.