It is sometimes taken for granted that Grotius included a short restatement of his doctrine of the free sea in his famous Introduction to the Jurisprudence of Holland (Inl. 2.1.16-17). This impression is incorrect, since Grotius merely restates a dictum of the Roman lawyer Marcianus (D. 1.8.2) and, in doing so, only deals with the condition of the sea according to private law. In Mare liberum, however, he employs Marcian's idea of the sea as a res communis in a different fashion. This article is concentrating on the use of arguments taken from private law in a debate on issues of public law in the light of the exceptions to Marcian's doctrine as formulated by medieval lawyers, which have culminated in the doctrine that states can indeed exercise authority over the sea. It is contended that Grotius had to argue along the same line by using arguments from private law in order to effectively refute that doctrine.