A debt is also owed to Simplicius, the commentator of Aristotle. He has given us accounts of Antiphon's attempt to square the circle, of the lunes of Hippocrates, and of a system of concentric spheres invented by Eudoxus to explain the apparent motions of the members of the Bolar system. He also wrote a commentary on the first book of Euclid's Elements, from which Arabian extracts were later made. Simplicius lived in the first half of the sixth century and studied at both Alexandria and Athens.