174. We must observe that the atom is not mathematically indivisible, for it has magnitude; it is, however, physically indivisible, because, like the One of Parmenides, it contains in it no empty space.[940] Each atom has extension, and all the atoms are exactly alike in substance.[941] Therefore all differences in things must be accounted for either by the shape of the atoms or by their arrangement. It seems probable that the three ways in which differences arise, namely, shape, position, and arrangement, were already distinguished by Leukippos; for Aristotle mentions his name in connexion with them.[942] This explains, too, why the atoms are called “forms” or “figures,” a way of speaking which seems to be of Pythagorean origin.[943] That they are also called φύσις[944] is quite intelligible if we remember what was said of that word in the Introduction (§ VII.). The differences in shape, order, and position just referred to account for the “opposites,” the “elements” being regarded rather as aggregates of these (πανσπερμίαι), as by Anaxagoras.[945] Footnote 940: The Epicureans misunderstood this point, or misrepresented it in order to magnify their own originality (see Zeller, p. 857, n. 3; Eng. trans. ii. p. 225, n. 2). Footnote 941: Arist. _de Caelo_, Α, 7. 275 b 32, τὴν δὲ φύσιν εἶναί φασιν αὐτῶν μίαν; _Phys._ Γ, 4. 203 a 34, αὐτῷ (Δημοκρίτῳ) τὸ κοινὸν σῶμα πάντων ἐστὶν ἀρχή. Footnote 942: Arist. _Met._ Α, 4. 985 b 13 (R. P. 192); cf. _de Gen. Corr._ 315 b 6. As Diels suggests, the illustration from the letters of the alphabet is probably due to Demokritos. It shows, in any case, how the word στοιχεῖον came to be used later for “element.” We must read, with Wilamowitz, τὸ δὲ Ζ τοῦ Η θέσει for τὸ δὲ Ζ τοῦ Ν θέσει, the older form of the letter Ζ being just an Η laid upon its side (Diels, _Elementum_, p. 13, n. 1). Footnote 943: Demokritos wrote a work, Περὶ ἰδεῶν (Sext. _Math._ vii. 137; R. P. 204), which Diels identifies with the Περὶ τῶν διαφερόντων ῥυσμῶν of Thrasylos, _Tetr._ v. 3. Theophrastos refers to Demokritos, ἐν τοῖς περὶ τῶν εἰδῶν (_de Sensibus_, § 51). Plut. _adv. Col._ 1111 a, εἶναι δὲ πάντα τὰς ἀτόμους, ἰδέας ὑπ’ αὐτοῦ καλουμένας (so the MSS.: ἰδίως, Wyttenbach; <ἢ> ἰδέας, Diels). Arist. Phys. Γ, 4. 203 a 21, (Δημόκριτος) ἐκ τῆς πανσπερμίας τῶν σχημάτων (ἄπειρα ποιεῖ τὰ στοιχεῖα). Cf. _de Gen. Corr._ Α, 2. 315 b 7 (R. P. 196). Footnote 944: Arist. _Phys._ Θ, 9. 265 b 25; Simpl. _Phys._ p. 1318, 33, ταῦτα γὰρ (τὰ ἄτομα σώματα) ἐκεῖνοι φύσιν ἐκάλουν. Footnote 945: Simpl. _Phys._ p. 36, 1 (Diels, _Vors._ p. 346), and R. P. 196 a. [Sidenote: The void.]