  According to different reports, Democritus ascribed the causes of things to necessity, and also to chance.  Probably the latter term should be understood as ‘absence of purpose’ rather than a denial of necessity (Barnes 1982, pp. 423–6).  Democritus apparently recognized a need to account for the fact that the disorderly motion of individual distinct atoms could produce an orderly cosmos in which atoms are not just randomly scattered, but cluster to form masses of distinct types.  He is reported to have relied on a tendency of ‘like to like’ which exists in nature: just as animals of a kind cluster together, so atoms of similar kinds cluster by size and shape.  He compares this to the winnowing of grains in a sieve, or the sorting of pebbles riffled by the tide: it is as if there were a kind of attraction of like to like (DK 68B164).  Although this claim has been interpreted differently (e.g. Taylor 1999b p. 188), it seems to be an attempt to show how an apparently ordered arrangement can arise automatically, as a byproduct of the random collisions of bodies in motion (Furley 1989, p.  79).  No attractive forces or purposes need be introduced to explain the sorting by the tide or in the sieve: it is probable that this is an attempt to show how apparently orderly effects can be produced without goal-directioned forces or purpose.