Possibilist knowledge is the kind of scientific knowledge about what makes something possible or impossible in a given context, given the satisfaction or absence of a necessary condition, and which is formed by possibilist laws and explanations.

Unlike knowledge of physical necessity, which states what must occur, or probabilistic knowledge, which indicates what is likely to occur, possibilist knowledge delineates the boundaries of what can and cannot happen. Possibilist laws are strict laws stating necessary conditions for events, and possibilist explanations show how apparently impossible or improbable phenomena could have occurred under specified conditions.

Building on new tools such as permissive and preventive conditions, this book rectifies longstanding problems in the philosophy of science, from the "how-possibly" explanations to mechanisms and negative causation, reconsidering the deductive-nomological model of explanation in a renewed light. Drawing examples from physics, biology, social sciences, psychology, and neuroscience, it demonstrates that possibilist reasoning is already pervasive, though unacknowledged, and essential for understanding multicausality and constraints in complex systems. In psychology, possibilist knowledge provides a methodological basis for unifying the discipline and reconciling scientific rigor with human freedom. More broadly, possibilism challenges the modern identification of science with determinism, showing that we can preserve exact science without assuming a deterministic universe. By offering a framework that compatibilizes scientific laws with the presumption of agency and responsibility, the book contributes to resolving one of the most enduring cultural conflicts: the tension between determinism, free will, and the authority of science.