In Metaphysics, Aristotle cites 10 antithetical pairs that were considered by some Pythagoreans as the principles of all things: “Others, among themselves, say that the principles are ten, those enumerated according to the series (of opposites): limit and unlimited, odd and even, unity and plurality, right and left, male and female, at rest and in motion, straight and curved, light and darkness, good and evil, square and rectangle” (ἕτεροι δὲ τῶν αὐτῶν τούτων τὰς ἀρχὰς δέκα λέγουσιν εἶναι τὰς κατὰ συστοιχίαν λεγομένας, πέρας καὶ ἄπειρον, περιττὸν καὶ ἄρτιον, ἓν καὶ πλῆθος, δεξιὸν καὶ ἀριστερόν, ἄρρρρεν καὶ θῆλυ, ἠρεμοῦν καὶ κινούμενον, εὐθὺ καὶ καμπύλον, φῶς καὶ σκότος, ἀγαθὸν καὶ κακόν, τετράγωνον καὶ ἑτερόμηκες, 986a 22). The fundamental antithesis is that between the “limit” and the “unlimited” (πέρας and ἄπειρον). All the other opposites are determined as its specifications. Πέρας (“limit”) refers to the delimitable (external) εἶδος(“form,” “figure”) of things, that which is accessible to knowledge. The unlimited (ἄπειρον) is, on the contrary, the diffuse, the uncircumscribable and undefinable, the matter that cannot be apprehended by knowledge.