Old English *motan and Middle English *moten, the ancestors of modern must, are commonly described as ambiguous between a possibility and a necessity reading. I argue instead that in the Alfredian Old English prose, *motan was a nonambiguous ‘variable-force’ modal, with the modal force different from both possibility and necessity. I propose that *motan’s variable-force effect was due to the presupposition of a collapse between possibility and necessity. Informally, motan(p) presupposed ‘if p gets a chance to actualize, it will’. I then trace the development of *motan into a modal genuinely ambiguous between necessity and possibility in Early Middle English.