Mare Liberum (known nowadays in English as The Free Sea) was published in 1609 in the form of a pamphlet. It only later on became clear that it was originally a chapter of a much larger manuscript: De Iure Praedae (On the Law of Prize and Bounty), the only surviving copy of which can be found in Leiden University’s Special Collections. The drafting and printing history behind the pamphlet and how it relates to the VOC is too complex to explain in a few words here. What is worth emphasizing however is that the concept of freedom of seas, the idea that the sea is a common heritage of mankind, free for all nations to use for trade and travel, as argued by Grotius in this pamphlet, is still one of the foundational principles of the international law of the sea nowadays.
Hugo de Groot, Mare liberum, book, 1609 [20643 F 18]