Melissus of Samos was a pre-Socratic philosopher from the ancient Greek island of Samos, active around the 5th century BCE. He is best known for his contributions to metaphysics and his defense of the concept of the One, arguing that reality is unchanging and indivisible. Melissus was a member of the Eleatic school, which emphasized the idea that change and plurality are illusions. His work is primarily known through fragments, where he challenges the views of earlier philosophers like Heraclitus, who believed in constant change. Melissus argued that if something exists, it must be eternal and unchanging, laying the groundwork for later philosophical discussions on existence and reality.