 The theory of suppositio is extended by the theory of “mobility” and “immobility”. If a particular mode of descensus is admissible, that type of supposition is called “mobile” (mobilis); otherwise it is called “immobile”  (immobilis).[15]  Heytesbury’s example of immobile supposition is the function of “penny” in “I promise you a penny” or “human” in “by necessity, every man is an animal” ([PC] 1494: fol. 188vb; [IHT] arg. 19; [SophAs] 398).