Abstract： Reasoning in daily life is defeasible. John Pollock proposed a defeasible reasoning view in response to the skepticism’s challenge to the possibility of knowledge acquisition. He also believes that knowledge is a collection of beliefs. It relies on defenses with time dimensions, balancing between reasons, and defeaters.Such defenses cannot be achieved by means of deductive inference models. Moral conflicts occur in the process of duties choices. The perception of moral obligations, especially the prima facie obligations manifests the defeasible nature. The traditional deductive mode of moral reasoning cannot explain the exceptional moral dilemma in moral conflict. Practices that rely on reasons for action are defeasible. Artificial intelligence that stems from the need for common-sense processing is the promoter of defeasible reasoning. The logical tools that characterize defeasible reasoning are not the traditional monotonous logic, but rather the non-monotonic logic.

Key words： defeasibility reasoning; epistemology; moral conflict; prima facie; monotonous