In the sixth century AD Simplicius produced detailed commentaries on several of the works of Aristotle, which help in our understanding of the "Physics", and of its interpretation in the ancient world. This is Urmson's translation of Simplicius' commentaries on "Physics 5" in which Aristotle lays down some of the principles of his dynamics and theory of change. What does not count as a change: change of relation?; the flux of time? There is no change of change, yet acceleration is recognized. Aristotle defines "continuous", "contact" and "next", and uses these definitions in discussing when we can claim that the same change or event is still going on.