  There are conflicting reports on whether atoms move in a particular direction as a result of their weight: a number of scholars have tried to reconcile these by supposing that weight is not intrinsic to the atoms, but is a result of the centripetal tendencies set up in the cosmic whirl (cf. O'Brien 1981; Furley 1989, pp. 91–102).  Atoms may have an inherent tendency to a kind of vibratory motion, although the evidence for this is uncertain (McDiarmid 1958).  However, their primary movement seems to result from collision with other atoms, wherein their mutual resistance or antitupia causes them to move away from one another when struck.  Democritus is criticized by Aristotle for supposing that the sequence of colliding atoms has no beginning, and thus for not offering an explanation of the existence of atomic motion per se, even though the prior collision with another atom can account for the direction of each individual atomic motion (see O'Keefe 1996).  Although the ancient atomists are often compared to modern ‘mechanistic’ theories, Balme warned of the danger of assuming that the atomists share modern ideas about the nature of atomic motion, particularly the idea that motion is inertial (Balme 1941).