The paper has three sub-topics: legal knowledge, legal norms, and evolutionary systems. The three are interconnected. A reflection on the nature of legal knowledge throws light on the nature of legal norms. Legal knowledge is largely a posteriori and it is so because norms are largely contingent. Being a realm of continual change, law has novelty as a fundamental feature. The process of legal change is not driven by chance but by the attempt to face ever new problems and changing circumstances. This supports a view of legal systems as adaptive and evolutionary, as classical pragmatism suggested. However, inference can give some a priori legal knowledge.