Although a copious body of research literature concerning the practice of adult self-directed learning exists, the theoretical dimension remains undernourished and poorly developed. If a theory for self-directed learning is to be developed, educators have to decide what is the critical element among the three words in the term self-directed learning or decide that the critical element is not one of the words but the synergetic consequence of combining the three words. Then theory-builders can address such questions as the following: (1) Is self-directed learning related to personality? How? Why? (2) Is self-directed learning developmental? Why? (3) Is self-directed learning behavior-variable? Why; and (4) Can self-direction in learning be enhanced or hindered? How? Why? Because the significance of such questions is embedded in theory, each can be traced backward into other theories of adults and children and horizontally to related theories of instruction and curriculum. By using such theories to analyze the answers to the questions asked earlier, information about self-directed learning can be converted to knowledge. Intrinsic motivation theory appears to provide some useful propositions for students of self-directed learning, and the idea of intrinsic motivation has even been used as a synonym for inquiry and self-directed learning. (The document includes a 12-item bibliography.) (CML)