τὸ νοητόν — "the thinkable" (from νοέω): the region of being that opens to the intellect rather than to the senses. On the "Divided Line" it forms the upper half alongside the visible (τὸ ὁρατόν) and is itself split in two: mathematical objects, to which discursive thought (διάνοια) is directed, and the Forms themselves, to which νόησις is directed; at its summit stands the Form of the Good as the "unhypothetical first principle" (ἀνυπόθετον), the cause both of truth and of intellect itself (Resp. VI 509d–511e). In Resp. VII 517b the prisoner's whole ascent from the cave is read as the soul's (ψυχή) passage into the "intelligible region" (νοητὸς τόπος), where the Form of the Good is "the cause of everything right and beautiful".
τὸ νοητόν is the proper object of the intellect (νοῦς): it is grasped by thought, not by sensation (αἴσθησις), and is therefore unchanging, whereas the sensible is always in becoming (Phaed. 79a). In the "Timaeus" it is precisely τὸ νοητὸν ζῷον that serves as the paradigm on which the Demiurge frames the visible cosmos, and the sharp distinction between the intelligible and the sensible (αἰσθητόν) is drawn at the very outset and in the doctrine of the third kind, space (Tim. 27d–28a; 51c–52a). Thus the soul's path to truth (ἀλήθεια) is an ascent into τὸ νοητόν: here knowledge (ἐπιστήμη) dwells, whereas opinion (δόξα) reigns over the sensible, and here too the soul's word (λόγος ψυχῆς) takes root, since only the intelligible admits steady speech.