Several other epistemologists (notably at Brown University, such as Ernest Sosa, and especially Roderick Chisholm), as well as his Arizona colleague Keith Lehrer, had written about defeasibility and epistemology.
In artificial intelligence, where non-monotonic reasoning had caused intellectual upheaval, scholars sympathetic to Pollock's work held him in great esteem for his early commitment and clarity.
Pollock's most direct pronouncement is the paper "Defeasible reasoning" in Cognitive Science, 1987, though his non-syntactic ideas were almost fully mature in Knowledge and Justification.
Although aided by a strong tail wind from AI and a few contemporary like minded philosophers (e.g., Donald Nute, Nicholas Asher, Bob Causey), it is certain that defeasible reasoning went from the obscure to the mainstream in philosophy because of John Pollock, in the short time between the publication of Knowledge and Justification and the second edition of Contemporary Theories of Knowledge.
Although OSCAR did not benefit from the contributions of a large number of professional programmers, it must be compared to CyC, Soar (cognitive architecture), and Novamente for its inventor's ambition.
Pollock described Oscar's main features as the ability to reason defeasibly about perception, change and persistence, causation, probabilities, plan construction and evaluation, and decision.
This work must be compared to Henry E. Kyburg's theories of probability, although Pollock believed that he was theorizing about a broader variety of statistical inferences.