Human agency has four irreducibly different dimensions - psychological, ethical, intellectual, and physical - which the traditional idea of a will tended to conflate. Twentieth-century philosophers criticized the idea that acts are caused by 'willing' or 'volition', but the study of human action continued to be governed by a tendency to equate these dimensions of agency, or to reduce one to another. Cutting across the branches of philosophy, from logic and epistemology to ethics and jurisprudence, Action, Knowledge, and Will defends comprehensive theories of action and knowledge, and shows how thinking about agency in four dimensions deepens our understanding of human conduct and its causes.

In Action, Knowledge, and Will, John Hyman ranges across the branches of philosophy, from logic and epistemology to ethics and jurisprudence, defends comprehensive theories of action and knowledge, and offers new answers to some of the most challenging theoretical and practical questions about human conduct, for example: What is the difference between the changes in our bodies we cause personally ourselves, such as the movements of our legs when we walk, and the movements we do not cause personally, such as the contraction of the heart? Are the acts we do to escape threats or fulfil obligations done voluntarily, out of choice? Should duress exculpate a defendant completely, or should it merely mitigate the criminality of an act? When we explain an intentional act by stating our reasons for doing it, do we explain it causally or teleologically or both? How does knowledge inform rational behaviour? Is knowledge a better guide to action than belief?