This chapter considers some of the philosophically more important kinds of necessity St. Thomas distinguishes and uses. These uses are not sharply distinct, but even the most clearly overlapping involve important differences of emphasis. The chapter also discusses Thomas making use of these distinctions in diverse areas as the impossibility of changing the past, the nature of the past infinity of time, and the sorts of necessity involved in the third way. It further considers Aquinas's modal vocabulary. While considering modal notions, it is important to be aware of the formal aspect of the terms, but also, and often more importantly, of their interpretation. In general, though with exceptions, philosophers have claimed that not even God can change the past, and in De Interpretatione and elsewhere Aristotle agrees. There is a certain amount of controversy concerning the correct interpretation of the De Interpretatione passages, but whether or not Aristotle intended this point at De Int.