In Blackfoot, a Plains Algonquian language spoken in Alberta, Canada, and Montana, USA, sentience, rather than telicity, is a primary determinant of argument structure. Subjects of transitive verbs, non-core objects of transitive verbs (benefactives, malefactives, sources, etc.), and primary objects of ditransitive verbs are all subject to a strict sentience requirement. This chapter follows Ritter and Wiltschko (2015) in assuming that the strict sentience requirements on argument structure are part of the grammar (i.e. part of the “narrow syntax”) of Blackfoot, and formalizes sentience as a feature that is subject to selection, a feature-checking operation, much like AGREE. This proposal correctly predicts that (a) not only agents but also causers must be sentient in Blackfoot; (b) sentient objects (not bounded ones) serve as both initiators and delimiters of events; (c) like event types, nominal types are distinguished by sentience, rather than boundedness; and (d) eventiveness is correlated with sentience, rather than dynamicity.