Euclid of Megara, a Greek Socratic philosopher who lived around 400 BC, founded the Megarian school of philosophy. Editors and translators in the Middle Ages often confused him with Euclid of Alexandria when discussing the latter's Elements. Most modern translations of Plato's Theaetetus render his name "Euclides."

Euclid was born in Megara, but in Athens he became a follower of Socrates. After Socrates's trial and death, Euclid returned to Megara, where other frightened pupils of Socrates found an asylum at his home. While none of his works have been preserved, he is quoted and discussed in various other ancient texts.

Euclid's philosophy was a synthesis of Eleatic and Socratic ideas. He identified the Eleatic idea of "The One" with the Socratic "Form of the Good," which he called "Reason," "God," "Mind," "Wisdom," etc. This was the true essence of being, and was eternal and unchangeable. As he said, "The Good is One, but we can call it by several names, sometimes as wisdom, sometimes as God, sometimes as Reason," and he declared, "the opposite of Good does not exist." While these doctrines may appear to contradict empirical reality, he argued that, since non-being cannot exist without becoming a species of being (i.e., no longer "non-being"), and since the essence of Being is the Good, the opposite of the Good cannot exist. His doctrinal heirs, the Stoic logicians, inaugurated the most important school of logic in antiquity other than Aristotle's peripatetics.

Euclid had three important pupils: Eubulides of Miletus, Ichtyas – the second leader of the Megarian school – and Thrasymachus of Corinth. This last one was the master of Stilpo, who was the master of Zeno of Citium, the founder of the stoic school.

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