Λέγω δὴ τὸ καὶ ὁποιανοῦν τινα κεκτημένον δύναμιν εἴτ᾿ εἰς τὸ ποιεῖν ἕτερον ὁτιοῦν πεφυκὸς εἴτ᾿ εἰς τὸ παθεῖν καὶ σμικρότατον ὑπὸ τοῦ φαυλοτάτου, κἂν εἰ μόνον εἰς ἅπαξ, πᾶν τοῦτο ὄντως εἶναι· τίθεμαι γὰρ ὅρον ὁρίζειν τὰ ὄντα, ὡς ἔστιν οὐκ ἄλλο τι πλὴν δύναμις.
I'm saying that a thing really is if it has any capacity at all, either by nature to do something to something else or to have even the smallest thing done to it by even the most trivial thing, even if it only happens once. I'll take it as a definition that those which are amount to nothing other than capacity.
Plato, Sophist, 247e, as translated by Nicholas P. White, in Plato: Complete Works (1997), p. 269
Plato, Sophist, 247e, as translated by Nicholas P. White, in Plato: Complete Works (1997), p. 269