Abstract: The current mainstream legal logic still focuses on deductive reasoning. As a logical tool for resolving deterministic problems, deductive reasoning, apart from addressing simple cases, mainly serves as a rhetorical argument in hard cases. For the decision-making in hard cases, we still have to rely on the plausible reasoning and related logical tools for dealing with uncertain problems. However, the current research on plausible reasoning mainly comes from empirical observations and lacks scientific explanations. Based on the principles of cognitive science, especially neural computation, attempts to explain the scientific essence of plausible reasoning by using the theory of "Bayesian Brain" and compares the similarities and differences between plausible reasoning and probabilistic reasoning, inference to the best explanation, pro/con argumentation and other logical tools. In the aspect of logical decision-making in hard cases, this paper agrees agrees with the statement that the essence of legal logic is plausible reasoning, and discusses the operation mode and guarantee of plausible reasoning in hard cases.