John L. Pollock (1940–2009) was an American philosopher known for influential work in epistemology, philosophical logic, cognitive science, and artificial intelligence.

Born John Leslie Pollock in Atchison Kansas, January 28, 1940, Pollock earned a triple-major physics, mathematics, and philosophy degree at the University of Minnesota in 1961. In 1965, his doctoral dissertation Analyticity and Implication at UC Berkeley was advised by Ernest Adams (making Pollock an intellectual descendant of Gottfried Leibniz and Immanuel Kant, through Ernest Nagel and Patrick Suppes). This dissertation contained an appendix on defeasible reasoning that would eventually blossom into his main contribution to philosophy.

Pollock took faculty positions at SUNY Buffalo, University of Rochester, University of Michigan, and University of Arizona. At Arizona, he helped found the Cognitive Science Program. He was an avid mountain biker and founded a riding club in Southern Arizona.

Knowledge and Justification, or simply KJ as it is known to at least two generations of philosophy graduate students, is the book that put John Pollock on the A-list of contemporary philosophers. It appeared at a time when American philosophy, and especially American epistemology, was obsessed with criteria and the analysis of what does it mean to know?. The Gettier problem, for example, was one of the most popular problems of the day: why is it that knowledge is not exactly "justified, true belief"? Pollock's book steps back from analytic criteria, which are presumably necessary and sufficient conditions. His epistemic norms are governed by defeasible reasoning; they are ceteris paribus conditions that can admit exceptions. Several other epistemologists (notably at Brown University, such as Ernest Sosa, Keith Lehrer, John Ladd, and especially Roderick Chisholm) had written about defeasibility and epistemology. But Pollock's book, which combined a broad scope and a crucial innovation, brought the ideas into the philosophical mainstream.