In the positivistic conception of law, sources of law (statute, precedent) are strictly distinguished from other legal materials such as doctrine. Courts as well as academia are, however, beginning to recognise the legal relevance of doctrine and case law of lower courts. This acceptance of such epistemic sources is justified within the rule of law if we recognise that courts and legislators do not create law in a vacuum, but rather build on a prior body of legal knowledge. This is relevant to legal argumentation and legal methodology, as it explicates that we have to take epistemic sources into account. This approach is supported by the recent epistemological recognition of testimony as source of knowledge. The epistemological approach to law offers an explanation to the recognition of precedents as source of law, and the authority of non-binding texts, thereby being suitable to a multi-level legal order where authority is more fluid than in traditional positivism.