The first step towards answering “what organ is in play when we perceive something in respect of both colour and sound?” is to show what is perceived in the situation in which we hear and see at the same time. To enable this Heidegger asks us to place ourselves in the imaginary situation of lying in a meadow, from which we can “see the blue of the sky, while simultaneously we hear the singing of the lark.” Asking how we perceive both together, the answer; we first of all perceive both as existing, i.e. as being, then that they are different from each other, “colour is one being, sound the other,” both the same as themselves, which also makes them countable. All that is thus perceived: “being, being one, different, both, the same, two, one, identity and non-identity,” is in addition to the colour and the sound, and are there as what Heidegger calls “an irremovable excess.” (135)
The identification of the excess, that which is there in addition to what is seen, heard etc. leads to the understanding that it is something that all our sensory perceptions, colours, sounds, tastes, touch have in common. Socrates then repeats his question: “with what sense-organs do you perceive this common element?” (138) This is asked because it has been shown that everything perceivable is perceived through a bodily organ. The excess, that which we perceive over and above what we see, hear etc, is common to all things, yet there is not a recognisable passageway to it, but there must be.
Dianoiein – inner perception
Theaetetus answers this question saying there is no special organ for perceiving the excess, the soul itself views the excess, while the sense organs give the soul access to colour, sound etc. Therefore the intrinsic duality of things, their physical properties and their being, are combined and seen in a unity by the soul, but which also has the capacity to separate them. This seeing of the excess by the soul is called dianoiein and is defined as the capacity “to look at a thing, thereby perceiving something about it,” a kind of inner perception. This seeing of the excess in dianoiein is “perceived so self-evidently and immediately that at first we do not pay the least attention to it.” (136)