It is clear, then, that we are entitled to refer the study of sums of series to Pythagoras himself; but whether he went beyond the oblong, and studied pyramidal or cubic numbers, we cannot say.[1]
49.Geometry and harmonics. It is easy to see how this way of representing numbers would suggest problems of a geometrical nature. The dots which stand for the pebbles are regularly called "boundary-stones" (ὅροι, termini, "terms"), and the area they mark out is the "field " (χώρα).[2] This is evidently an early way of speaking, and may be referred to Pythagoras himself. Now it must have struck him that "fields" could be compared as well as numbers,[3] and it is likely that he knew the rough methods of doing this traditional in Egypt, though certainly these would fail to satisfy him. Once more the tradition is helpful in suggesting the direction his thoughts must have taken. He knew, of course, the use of the triangle 3, 4, 5 in constructing right angles. We have seen (p. 20) that it was familiar in the East from a very early date, and that Thales introduced it to the Hellenes, if they did not know it already. In later writers it is actually called the "Pythagorean triangle." Now the Pythagorean proposition par excellence is just that, in a right-angled triangle,
- ↑ Speusippos (cf. p. 102, n. 2) speaks of four as the first pyramidal number; but this is taken from Philolaos, so we cannot safely ascribe it to Pythagoras.
- ↑ Proclus, in Eucl. I. p. 136, 8, ἔστι δὲ τὸ ὄνομα (sc. ὅρος) οἰκεῖον τῇ ἐξ ἀρχῆς γεωμετρίᾳ, καθ' ἣν τὰ χωρία ἐμέτρουν καὶ τοὺς ὅρους αὐτῶν ἐφύλαττον ἀσυγχύτους. We have ὅροι of a series (ἔκθεσις), then of a proportion, and in later times of a syllogism. The signs :, ::, ∴ seem to be derived from this. The term χώρα is often used by the later Pythagoreans, though Attic usage required χωρίον for a rectangle. The spaces between the γραμμαί of the abacus and the chess-board were also called χῶραι.
- ↑ In his commentary on Euclid i. 44, Proclus tells us on the authority of Eudemos that the παραβολή, ἔλλειψις and ὑπερβολή of χωρία were Pythagorean inventions. For these and the later application of the terms in Conic Sections, see Milhaud, Philosophes géomètres, pp. 81 sqq.
περιτιθεμένων ἑτερομήκεις καὶ ἄνισοι πάντες ἀποβαίνουσιν, ἴσως δὲ ἰσάκις οὐδείς. It will be observed that Aristotle here uses εἶδος in the sense of "figure." The words καὶ χωρὶς apparently mean χωρὶς τοῦ ἑνός, i.e. starting from 2, not from 1.
