John Philoponus, Commentary on Aristotle's De Anima 68.3 & 11-13 Hayduck, here commenting on De anima 406a6. Philoponus' own source is probably Aristotle, Metaphysics 985b4-22 (web address 1), where Aristotle describes the basic principles of the atomistic philosophy of Leucippus and Democritus. Aristotle says that they posit the full (i.e. the atoms) and the empty (i.e. the void) as the principles, and that their differentiations are the cause of all change. The differentiations of the atoms are 'rhythm' (which Aristotle glosses as 'shape'), 'contact' (which Aristotle glosses as 'arrangement'), and 'turning' (which Aristotle glosses as 'position'). These differences Aristotle illustrates as follows: A differs from N in shape (i.e. rhythm), AN from NA in arrangement (i.e. contact), and Z from N in position (i.e. turning).
[1] Abdera (alpha 26) is specified because of Democritus; otherwise 'Ionic dialect' would have made the point.
[2] For Democritus see generally delta 447, delta 448.
[3] The latter has its own entry at delta 562 (headword only).