 Since Aristotle it was standard practice to note that ‘possible’ can be used in two ways. In the first ‘possible’ is compatible with ‘necessary’, but in the second, that which is necessary is not (merely) possible. This second way is usually called ‘contingent’. Sherwood mentions this distinction, but goes on to say that he will be using it in a broader sense, in which possibility is compatible with necessity (Kretzmann 1966, p. 41). However, he makes a further distinction, which he does maintain, between the two ways that ‘impossible’ and ‘necessary’ can be used. These different ways are expressed in temporal notions: