to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their place..." The ship had to be maintained in a seaworthy state, for, in return for Theseus's successful mission, the Athenians had pledged to honor Apollo every year henceforth. Thus, the Athenians sent a religious mission to the island of Delos (one of Apollo's most sacred sanctuaries) on the Athenian state galley—the ship itself—to pay their fealty to the god. To preserve the purity of the occasion, no executions were permitted between the time when the religious ceremony began to when the ship returned from Delos, which took several weeks. To preserve the ship, any wood that wore out or rotted was replaced; it was thus unclear to philosophers how much of the original ship remained, giving rise to the philosophical question of whether it should be considered "the same" ship or not. Such philosophical questions about the nature of identity are sometimes referred to as the "Ship of Theseus" paradox. Regardless of these issues, the Athenians preserved the ship. They believed that Theseus had been an actual, historical figure and the ship gave them a tangible connection to their divine provenance. Theseus and Pirithous Theseus's best friend was Pirithous, king of the Lapiths. Pirithous had heard stories of Theseus's courage and strength in battle but wanted proof so he rustled Theseus's herd of cattle and drove it from Marathon and Theseus set out in pursuit. Pirithous took up his arms and the pair met to do battle but were so impressed with each other's gracefulness, beauty and courage they took an oath of friendship and joined the Calydonian boar hunt. In Iliad I, Nestor numbers Pirithous and Theseus "of heroic fame" among an earlier generation of heroes of his youth, "the strongest men that Earth has bred, the strongest men against the strongest enemies, a savage mountain-dwelling tribe whom they utterly destroyed." No trace of such an oral tradition, which Homer's listeners would have recognized in Nestor's allusion, survived in the literary epic. Later, Pirithous was preparing to marry Hippodamia. The centaurs were guests at the wedding feast, but got drunk and tried to abduct the women, including Hippodamia. The Lapiths won the ensuing battle. In Ovid's Metamorphoses Theseus fights against and kills Eurytus, the "fiercest of all the fierce centaurs" at the wedding of Pirithous