Fregean Extensions of First-Order Theories

It is shown by Parsons [2] that the first-order fragment of Frege's (inconsistent) logical system in the Grundgesetze der Arithmetic is consistent. In this note we formulate and prove a stronger version of this result for arbitrary first-order theories. We also show that a natural attempt to further strengthen our result runs afoul of Tarski's theorem on the undefinability of truth. Mathematics Subject Classification: 03B10.