The doctrine of sources of law is a cornerstone of argumentation as it draws the boundary between what is law and what is not. Knowledge of valid law is acquired through analysis of sources of law. Theory of legal argumentation has been one of the focal areas in Finnish and European legal theory since the 1970s. In this article professor Aulis Aarnio，one of the developers of that theory，presents his latest views on the classifications and structures of legal argumentation especially with regards the doctrines about the sources of law. The article is significant as it seeks to adapt the doctrines of the sources of law to the increasingly important phenomenon of law beyond the state. Finnish law adopts basically the dualistic view that international law needs to be transposed to domestic law for it to become applicable in the domestic courts. EU law，for instance，increasingly gets importance directly，which renders it necessary to take such phenomena into account in the doctrines of sources of law themselves. The doctrine on the sources of law also still enables legal theorists to express and present the view about law in a specific country，such as Finland.