While retrieval practice has been studied for over a century, it has gained a significant body of research within the past fifteen years in cognitive psychology and the learning sciences. However, there is little discussion in its study and application within topics of adult learning. In this article, we examine the historical and philosophical foundations of self-directed learning and retrieval practice and advocate for functional contextualism as a philosophy of science for self-directed learning research. Functional contextualism provides a synthesis between the norms of humanism and the scientific focus of cognitivism. Particularly, we look at retrieval practice as a potential area of research within self-directed learning, viewing it as an 'act-in-context'. We hope this article reinvigorates research into self-directed learning, particularly the investigation of empirically-based self-guided learning strategies, the facilitation of metacognition for self-directed adult learners, and the adoption of a functional contextual approach to self-directed learning and other adult learning topics.