 One observed difference between some triggers is that they don’t seem to reliably generate presuppositions in certain contexts. As we mentioned in Section 6, theories of presupposition projection tend to adopt the working hypothesis that presuppositions are associated with expression types: if an expression α triggers a presupposition φ, then φ will always be triggered by α. This is just a matter of economy: since a projection theory already provides a mechanism for explaining how presuppositions, once triggered, can seem to disappear, there is no need for supposing that there are part-time presupposition triggers. Indeed, it would be a bit of a nuisance if it turned out that that some expressions or constructions trigger their presuppositions on a part-time basis, because then we would have two ways of explaining why a given presupposition fails to appear: echoing our discussion in section 6, either it has been canceled (say through being resolved in an embedded context) or it wasn’t triggered in the first place.