 The appeal to formal laws and deductive relations is dropped for sets of concepts or vocabularies in the replacement analysis (Spector 1978). This approach allows for talk of entity reduction or branch reduction, and even direct theory replacement without the operation of laws, and circumvents vexing difficulties raised by bridge principles and the deductive derivability condition (self-reduction, infinite regress, etc). Formal relations only guarantee, but do not define, the reduction relation. Replacement functions are meta-linguistic statements. Like Sellars had argued in the case of explanation, this account distinguishes between reduction and testing of reduction, and highlights the role of derivations in both. Finally, replacement can be in practice or in theory. Replacement in practice does not advocate elimination of the reduced or replaced entities or concepts (Spector 1978).