This research deals with how extraction rules are constrained in cases where their unconstrained application would give rise to semantic ambiguity. Of particular concern is the application of extraction rules to noun phrases (NP's) where word order is the only indication of the different syntactic functions of two adjacent NP's. Samples from German, Icelandic, Tzotzil and Navajo are examined in a search for the universal basis for the constraint. Cross-linguistically, the universal constraint is independent of language-particular word order. It is dependent on the subjecthood/definiteness correlation. Subjects must be definite and objects indefinite to be extractable. This constraint is a functional one; it uses universal linguistic tendencies to construct a perceptual strategy for the univocal comprehension of what would otherwise be ambiguous. Any meaningful formulation of this constraint must be able: (1) to state the constraint in terms of grammatical relations rather than in terms of word order, and (2) to deal explicitly with the teleological nature of the constraint. When a language needs to develop a constraint to avoid ambiguity, it can freeze universal tendencies into requirements. This constraint may contribute a possible definition of universal subjecthood. (PMJ)