When Grosseteste wrote On Free Decision modal concepts were typically understood in terms of the notion of change and thus of time. Thus, Grosseteste speaks of a necessary proposition, in this sense, as one that is true and cannot become false, or as one whose truth is unable to cease to be. A contingent proposition can change its truth value; a possible proposition, if false, can become true, and an impossible proposition is false and cannot become true. Conceptions of modality along these lines were sometimes described as per accidens conceptions, since in many cases a proposition’s modal status could change with the passage of time; its modal status, as it were, happens to or is incidental to it (accidit).  “Caesar crossed the Rubicon,” for example, was once false and not necessary, but became true and necessary, because unable to become false, after Caesar crossed the Rubicon. Indeed, true propositions about the past were the standard examples of propositions necessary in the sense that their truth cannot cease. 