Our question and task are once more established. What organ is in play when we perceive something in respect of both colour and sound? To answer this question we must first show what is perceived in this situation and how this perceiving itself must be.
The inquiry (185 a 8-186 c 6) proceeds in four clearly distinguishable steps (A-D).
The first step continues until 185 b 6. Let us imagine ourselves in a quite ordinary situation, where we 'hover', so to speak, within an immediate perceiving simultaneously of colour and sound. Lying in the meadow we see the blue of the sky, while simultaneously we hear the singing of the lark. Colour and sound reveal themselves to us. We perceive both. What do we then perceive in respect of both? What can we perceive here?
We shall let Socrates ask this (185 a 8):
Περὶ δὴ φωνῆς καὶ περὶ χρόας πρῶτον μὲν αὐτὸ τοῦτο περὶ ἀμφοτέρων ἦ διανοῇ, ὅτι ἀμφοτέρω ἐστόν;
[Theaetetus] Ἔγωγε.