This paper proposes an underlying representation for passive sentences in Mojave and in Uto-Aztecan, and explores the broader issues that arise in extending the analysis to other languages and incorporating it in linguistic theory as a substantive language universal. In the introduction, this underlying representation is presented and discussed in relation to previous analyses of the passive. It is claimed specifically that passive sentences in Mojave and Uto-Aztecan are basically impersonal, and derive from structures in which a clause with unspecified subject is embedded as subject complement to the predicate be. Agentive phrases, when they occur, are said to derive from an external source, and are not considered an intrinsic part of the passive construction. The Uto-Aztecan evidence, primarily comparative and diachronic, is presented in §1. It is argued that reflexive constructions commonly assume passive function because both involve ‘non-distinct’ arguments, of which coreferential and unspecified arguments are special cases. The concept of non-distinct arguments receives considerable support from various syntactic changes that occurred in Uto-Aztecan. The Mojave evidence, primarily synchronic, is presented in §2. In §3, a variety of related issues are treated, including potential problems in extending the analysis to English, the source of by-phrases, the nature and status of unspecified arguments, the semantic claims implied by the proposed underlying representation, and the relationship between passive and perfective constructions.