A philosphical framework to explain the value of using activity as a treatment method is the challenge pursued for this lectureship. Primary resources came from Soviet psychology, the only social science discipline using the concept of activity as a focus of study. The focus of study selected for occupational therapy is disability, which is explained within the context of doing an activity. The patient’s purpose for doing an activity is described by the degree of sensorimotor thought processed during a functional state. A hierarchy of activity analysis is used to begin the development of typologies for feasible operations, satisfactory results, and desirable activities. The philosophical framework is applied to three types of patient populations that pose problems in stating treatment objectives, patients that have (a) a good prognosis but one that is associated with alternative explanations for change, (b) a poor prognosis associated with a lifelong disability, and (c) a grave prognosis associated with a progressive loss of functional abilities. A refined treatment hypothesis is suggested: Therapeutic activity compensates for disability by using remaining capabilities to accomplish desirable activities with satisfactory results.