the intermediate
The intermediate (τὸ μεταξύ) is what lies between two poles without coinciding with either. In the Republic it is a distinct region of being: between what purely is (τὸ ὂν εἰλικρινῶς) and what in no way is (τὸ μὴ ὄν) Plato places that which «both is and is not», and it is upon this that opinion (δόξα) is set, whereas knowledge (ἐπιστήμη) is directed at being and ignorance (ἄγνοια) at not-being (Resp. V 477a, 478d). The many beautiful things, unlike the beautiful itself, «roll about somewhere» between being and not-being (Resp. V 479d); and to each power its own object is assigned — «the extremes to the extremes, the intermediate to the intermediate» (Resp. V 478e).
In the Symposium τὸ μεταξύ is the mode of being of a mediator: right opinion (ὀρθὴ δόξα) is something between «wisdom and ignorance» (μεταξὺ σοφίας καὶ ἀμαθίας), for it hits the truth yet cannot «give an account» (Symp. 202a); and everything daemonic (τὸ δαιμόνιον), above all Eros, is «intermediate between god and mortal» (μεταξὺ θεοῦ καὶ θνητοῦ) and binds the two (Symp. 202e). The intermediate is not a contrary nor a pole on a scale, but an independent position between the poles: δόξα is intermediate between ἐπιστήμη and ἄγνοια, not opposed to knowledge (Resp. V 477a). In this τὸ μεταξύ differs from the pair «truth/falsehood»: it does not negate one pole but keeps a share (μέθεξις) in both.
In the Phaedo, between any two opposites lie two processes of becoming (γενέσεις), and upon this «between» rests the proof of immortality from the mutual passage of opposites (Phd. 71a). In the Philebus, between the unlimited (ἄπειρον) and the one (ἕν) lie the determinate numbers that must be grasped (Phlb. 16e), while in the Gorgias τὰ μεταξύ are things «neither good nor bad», a mean between good and evil (Grg. 467e).