In view of the strong influence of Dewey's thinking on contemporary educational thought, looking back over his epistemological conceptions is of crucial importance. The heart of Dewey's theory of knowing rests on a fundamental postulate derived from his naturalistic interpretation of human cognitive development: that of the functional separation, in the understanding of meaning, between observed or experienced phenomena and theoretical constructs. This postulate underpins Dewey's agreement with operationalism, his critique of the spectator theory of knowledge and his conception of causality as a sequential order. If this postulate is disproven, the principles relating to intellectual training that are derived from Dewey's theory of knowing collapse.