Abstract

The atheist who begins to argue his case by saying, ‘Suppose there is an omniscient Being of the sort in which Christians believe ...’ is employing a very familiar move in argumentation. However, most books on argumentation theory ignore ‘suppositions’ completely. Searle omits suppositions entirely from his taxonomy of speech acts and this appears to lead to a similar omission in Speech Acts in Argumentative Discussions by van Eemeren and Grootendorst.This paper argues that ‘suppositional argument’ is elegant, powerful and extremely common, that the correct way to understand it is based on Gottlob Frege's distinction between ‘asserted’ and ‘unasserted’ propositions and hence that suppositions are neither assertions nor (and this is more important) assertives. The paper discusses the connections between suppositions and conditionals; it argues that argumentation theory which ignores suppositions is systematically misleading; and it concludes by indicating some possible developments in argumentation theory