 References for the various sciences are, generally identifiable: Euclid, Nicomachus and sometimes even Archimedes for mathematical tenets (Epistles 1, 2 and 6); Ptolemy supports astronomy and geography (Epistles 3 and 4); the Pythagoreans and again Nicomachus are the sources for musical theory (Epistles 5 and 6). The Greeks already saw arithmetic and geometry as the necessary, though not unique, means of attaining philosophical knowledge.