This chapter explains Aristotle’s claim that time measures change, and is measured by change. It argues (against Julia Annas) that Aristotle does not equate measuring and counting. His usual view (for example, in Metaphysics X) is that counting is a special type of measuring that can be used to find the quantity of collections of things. This provides extra evidence for the view (defended in chapter five) that when Aristotle defines time as a kind of number, he is using the word ‘number’ in a special sense. In order to measure time, it is necessary to find an appropriate unit. Aristotle thinks that the appropriate unit is a certain change: the revolution of the outermost sphere of the heavens. He thinks it is prima facie puzzling, given that time is not a kind of change, that a unit of change can be used to measure time. The interpretation in this chapter explains why he thinks this is puzzling, and how he can solve the puzzle by appealing to his view that time is a number of change.
Keywords: measure, change, counting, measuring, Metaphysics, number, unit, Julia Annas