Among the great philosophers who treated on music, =Aristotle= (384 B. C.) holds an important place. We find his theories expressed in one of his works called “Problems.” A pupil of his—=Aristoxenus= (350-320 B. C.), has left the most valuable treatise on music, of any of the ancients, the oldest musical work known at the present time; it is, unfortunately, not complete. Aristoxenus was a practical, in addition to being a theoretical musician; he thought that the ear was the final court of appeal in matters musical. Hence the musical world was divided into two factions: the Pythagoreans, who held that music was purely a matter for arithmetical investigation, and the Aristoxenians, who claimed that the chief end of music was to be listened to. This dispute lasted for many centuries. =Boethius=, the Roman philosopher, in his writings takes sides with the Pythagoreans and pours contempt on the mere musician. The successors of the Pythagoreans are even yet not extinct, as every now and again some wiseacre turns up with a scheme to secure just intonation, at the price of losing all that music has gained under our present system. =Plato= (430 B. C.), the greatest of philosophers, has much to say about music; but these sayings are largely incomprehensible to modern understandings. =Euclid= (323 B. C.), the great mathematician, treated largely of music.[5] =Aristides Quintilianus= was another author of great weight. =Plutarch=, in his _Symposia_, has one devoted to music, but unfortunately the meaning of these authors is often so obscure that it cannot now be discovered. Alexandria, in Egypt, came into prominence in music when the great library was founded there by Alexander the Great, in 332 B. C. =Eratosthenes= (276-196 B. C.), the librarian, figures in the mathematics of music. When we reach the Christian Era, we meet with two more writers, =Didymus= (A. D. 60), who introduced the “minor”[6] tone into the scale, and =Claudius Ptolemy= (A. D. 130).