 Questions of identity through time are more often difficult, because they involve issues of persistence. If a kitten grows up it ceases to be a kitten but it is the same cat and the same animal. But if we tear down our garage and build a tree house out of the lumber it is not the same building. When does something survive change and when not? Which kinds of changes? Sortals providing criteria for continued existence through time (or so-called “persistence criteria”) are often referred as “substance sortals”. By their features, essential sortals will clearly qualify as substance sortals but not necessarily vice-versa. Similarly to the case of essential sortals, there are systematic disagreements among philosophers whether the substance sortals are to be located at a relatively specific level, e.g., “dog”, “river” or at the level of a more general category, e.g., “animal”, “geographic feature”.