knowledge, science
Knowledge in the strict sense — stable cognition of what is, distinguished from opinion (δόξα) and perception (αἴσθησις). In the Republic ἐπιστήμη is directed at being (τὸ ὄν) (Resp. V 477a, 478a); only dialectic (διαλεκτική), ascending to the unhypothetical first principle, is knowledge in the full sense (Resp. VII 533e).
The whole Theaetetus is a testing of definitions of ἐπιστήμη: perception (Tht. 151e), true opinion (187b), true opinion with an account (μετὰ λόγου, 201d) — all are rejected. In the Meno true opinion guides right action no worse than knowledge, yet it is held in the soul unreliably — like the «statues of Daedalus» it «runs away» unless bound by «reasoning about the cause» (αἰτίας λογισμός), which turns it into knowledge (Men. 97a). In the Phaedo genuine knowledge is recollection (ἀνάμνησις) of what the soul saw before birth (Phd. 75d).
Its opposites are opinion (δόξα), which gives no account of essence; ignorance (ἄγνοια), directed at what is not; falsehood (ψεῦδος). Its supporting ties are truth (ἀλήθεια), intellect (νοῦς), form (εἶδος), account (λόγος): knowledge is the grasp of forms in themselves (καθ’ αὑτά), able to give a λόγος of them. In the strict sense ἐπιστήμη belongs to dialectic alone (Resp. VII 533c).