This article explores the architecture of the interface between morphosyntax and lexical semantics, in particular the semantic underpinnings of argument realization. Many theories of lexical meaning assume that argument realization is derived from underlying event structure: the relative prominence of coarguments in a clause follows from their relative semantic prominence in how the event unfolds. I show that event structure is not sufficient to capture certain generalizations about argument realization, however, focusing on arguments that alternate between direct and oblique realization. I show that for these alternations the relevant semantic contrast is in strength of truth conditions: direct realization encodes a monotonically stronger set of truth conditions associated with the alternating argument than oblique realization. This, I suggest, follows if word meanings are built from basic units that are related to one another implicationally, and the relative implicational strength of such components figures into argument realization. I use as a case study English locative and conative alternations, which, I argue, reflect stronger and weaker degrees of affectedness along an independently motivated affectedness hierarchy. I also show that similar contrasts are found with other alternations on other hierarchies. I conclude by suggesting that a theory of weakening truth conditions is not incompatible with event-structural analyses of verb meaning, and in fact the two augment one another.