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How did Grotius’s writings defend freedom of the seas?

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Painting by Rembrandt showing Aristotle in rich dark clothing with a gold chain, resting his hand thoughtfully on a marble bust of Homer, illuminated against a dark background. — natural law *Aristotle Contemplating the Bust of Homer*, oil on canvas by Rembrandt van Rijn, 1653; in the collection of the Metropolitan Museum of Art, New York.

Hugo Grotius defended the freedom of the seas by arguing that the ocean is a natural resource that cannot be subject to the territorial sovereignty of any nation. Commissioned by the Dutch East India Company to legally justify the seizure of a Portuguese vessel, Grotius contended that Spain and Portugal’s claims to a trade monopoly in the East Indies were illegitimate because they deprived other nations of their natural trading rights.1

His arguments were published in 1609 as Mare Liberum ("The Freedom of the Seas"), a work rooted in the philosophy of natural law. Grotius posited that certain principles of natural justice and moral responsibility are inherent in human reason and immutable, serving as a necessary check against the arbitrary exercise of power by sovereign states. By asserting that the high seas were open to all, he provided a foundational framework for international law that challenged the exclusive claims of powerful empires and supported the interests of smaller, trading-focused nations.12