  If a changing thing has different and incompatible properties then a contradiction is threatened. The obvious move to make when confronted with the fact that things change, is to say with Kant (1781) that they change in relation to time, which avoids the inconsistency. But then another problem emerges. In what sense can one thing persist through change? Identity across time and space is the mark of universals, but we also account particulars such as billiard balls and persons as having self-identity across time.