ADELARD or AETHELARD of Bath (12th century), English scholastic philosopher. He studied in France and travelled in Spain, Italy, North Africa and Asia Minor, where he became acquainted with Arabian scholarship. He returned to England in the time of Henry I., and received an annual grant from the revenues of Wiltshire. Adelard translated Euclid's Elements, probably from an Arabic version. The work, which was published in Venice in 1482 under the name of Campanus de Novara, at once began to be used as a text-book in the schools. Adelard's special contribution to scholastic philosophy was the treatise De eodem et diverso, in which he enunciated the theory of "indiffer ence." See SCHOLASTICISM.
It is in the form of a dialogue between himself and his favour ite nephew, and was dedicated to Richard, bishop of B4yeux from 1113 to 1133. He wrote also a group of treatises on the astrolabe (a copy of which is in the British Museum), on the abacus (three copies exist in the Vatican Library, the library of Leyden University and the Bibliotheque Nationale in Paris), translations of the Kharismian Tables and an Arabic Introduction to Astronomy.