On close inspection, Themistius’ rephrasing of two passages in Physics IV 2, in which Aristotle compares Plato’s Timaeus and the ‘unwritten doctrines’, suggests that he was guided in his understanding by a loose association with the account of Plato’s Ideas in the Metaphysics. Themistius also interprets Aristotle’s remarks about ‘place’ in Timaeus as pointing to the main feature of ‘prime matter’, namely indetermination.