When Piaget ascribed the origin of knowledge to action, he distanced his theory from most other theories of the origins of mind. As a result, Piaget's conception of mental action has been quite controversial. Piaget's recent, functionalist revisions emphasize procedures and their role in the development of structures. In the new view, structures have their origins in procedures, not simply in actions alone. When mental actions are defined as "signifying implications," they are logical. Though mental actions are not physical, they are also neither linguistic nor perceptual. Dissatisfied with the emphasis he had placed on truth-table extensional logics, Piaget later sought a new theory of meaning based on intensional logic, a logic entailing implications. Piaget's discussion of protologics, the nascent logical forms that underlie sensorimotor activity, indicates that even though functions are emphasized, structures are not fully rejected. The nature of the logical system Piaget alludes to is based on implications and inferences, and the logic of meaning so elaborated is the preparation for operatory logic. Piaget's new functionalism also posits new sources of disequilibrium, emphasizes contradictions in cognitive systems, considers the role of negation in cognitive conflict, and offers revisions of the conception of equilibration and the vocabulary of Piagetian concepts. (RH)