 As the disputed question as a form began to fade in importance in the theological faculty, it was replaced by variations on the disputation form in the arts faculty, focusing on questions of logic and natural philosophy. Of these three types of literature which become more significant in the 13th and 14th centuries, obligatio is the only one that unambiguously refers to a form of argument. Sophismata and insolubilia can refer either to the propositions that might be discussed in a debate or treatise or other form, or a form of argument for discussing these types of propositions. The literature concerning these kinds of problems ranges from formal disputes on the propositions which attempt to solve or avoid the problem posed by the statements to treatises or rules about how to solve the puzzles the disputes over them reveal. As a type of proposition, sophismata are ambiguous propositions about which arguments might plausibly be given both that they are true and that they are false. They are, as Ashworth puts it, “the medieval ancestor of ‘the morning star is the evening star’,” that is, a proposition epitomizing a larger problem (Ashworth 1992, 518). Insolubilia are propositions that are either very difficult or impossible to hold as true or false. These propositions are usually self-referential paradoxes such as the liar paradox (e.g., ‘everything I say is false’). The influential view of William Heytesbury was that insolubles should be resolved in the context of an obligatio, a specialized form of disputation, and Heytesbury proposes rules for solving insolubilia as well as sophismata in this way (Spade 1982, 252). (For a catalog of insolubilia literature, see Spade 1975). 