This paper offers a discourse-pragmatic account of the constraint on indefinite DPs as subjects of specificational copular clauses (a doctor is Mary). Building on Mikkelsen's (2004) proposal that specificational subjects are topics, I argue that they must be contrastive topics which properly contain F-marked constituents. I show that this can account for the absolute ban on simple indefinite subjects, and allow for more complex indefinites to be subjects. Finally, I discuss the syntactic analysis that would be predicted given my pragmatic analysis, and the puzzles that arise from it.
