January 14: World Logic Day

When the Athenian hero Theseus left on his mission to kill the Cretan Minotaur, he promised his father, king Aegeus, he would raise white sails on his ship if he survived. But he didn’t (accidentally or not), and when his father saw his black-sailed ship entering the harbour, he jumped from a cliff, giving his name to the Aegean sea. Theseus took over the kingdom, and his ship became an exhibition piece. But over time, it needed multiple repairs, and this raised a question: how much material could be replaced before it would stop being “Theseus’ ship”? It was one of those theoretial issues the ancient Greeks called “paradoxes”, meaning “contrary to accepted opinion”, and philosophers discussed them endlessly. Herakleitos stated that a man never steps in the same river twice, for it’s not the same water, nor the same man. And Zeno famously declared that Achilles never could have overtaken a tortoise, since he had to reach its starting point first, in endless repetition. Most famous paradox of all? Eubulides: “What I’m saying now, is a lie.”