 Identity over time is itself a controversial notion, however, because time involves change. Heraclitus argued that one could not bathe in the same river twice because new waters were ever flowing in. Hume argued that identity over time was a fiction we substitute for a collection of related objects. Such views can be seen as based on a misunderstanding of Leibniz’s Law: if a thing changes something is true of it at the later time that is not true of it at the earlier, so it is not the same. The answer is that what is true of it at the later time is, say, “being muddy at the later time”, which was always true of it; similarly, what is true of it at the earlier time, suitably expressed, remains true of it. But the question remains how to characterise identity through time and across change given that there is such a thing.