The ideas about possible states and processes introduced in Chap. 10 are developed further in this chapter. The interplay between possibility and time is developed, keeping track of the time when a possibility arises and the time when it is possible for a quantity of have a property or be related to something. The connection with possible processes facilitates the analysis of the implicit modal import of predicates describing the condition of quantities of matter such as the warmer than relation, which involves a possible heating process. Many thermodynamic properties involve an implicit modal import along the same lines. The relation of modality to time raises issues about the mereological structure of states. This leads to the suggestion that a notion of possible world can be seen as a total sum of states. This provides a basis for bringing together the ideas about modality in Chap. 3 with those in these later chapters.