When a Semantic Web application encounters a new URI in an RDF statement, how should it determine what resource that URI is intended to denote, and learn more about it? Since assertions are the currency of Semantic Web applications, in practical terms this question can be viewed as asking: What additional assertions should be used if the application wishes to learn more about the URI's denoted resource, and what mechanism should be used to find those assertions? This paper compares two architectural approaches from this perspective: one based on the notion of URI declarations, the other based on a marketplace of competing definitions. It argues that the URI declarations approach offers more desirable architectural characteristics for the Semantic Web, largely because it reduces URI collision. Key words: Semantic Web, RDF, identity, URI declaration, URI definition