The first answer that Theaetetus offers in response to Socrates’ question: “what is knowledge?” is that knowledge is perception (aisthesis). Heidegger then asks how this answer is arrived at, what is it in perception that appears to link it to knowledge, understood as the “possession of truth,” and thus to being.
For the Greeks the becoming perceived of something in perception is the same as phantasia. This is what a thing appears as, what it shows itself as, its appearance. Accordingly that which we perceive in a perception is there in a state of being perceived, which we take for what it presents itself as. Heidegger gives an example: “the moon itself that appears in the sky, that presents itself and is present; this is something that shows itself.” The moon shows itself as being present, which we take for what it shows itself as, the moon. Therefore aisthesis has a double meaning relating to the perceived in its perceivedness, and the perception in which perceivedness occurs.
Therefore Theaetetus’ thesis is that knowledge, as truth, is this perceivedness of what is seen. That which we perceive in a perception is there before us as something that shows itself as present, as “a kind of unhiddenness.” Perception, aisthesis, is equated with truth as unhiddenness, aletheia, because it “appears the most immediate mode of the unhiddenness of something…the most tangible truth.”
Yet it is still open to question that what becomes manifest in this relationship, between what shows itself and perception, is being. For this to be the case our “perceptual comportments,” seeing, hearing etc, through which we have a perception, must “have a relationship to beings as such.” To test whether or not this is the case Heidegger’s analysis focuses on the essence of perception itself, asking whether or not a perceptual comportment “can bring itself into a relationship to beings as beings, such that the unhiddenness of beings is given in the perceivedness occurring in such a comportment.” (121)