[1] After the dialogue between Socrates and Phaedrus has essentially come to a conclusion in Phaedrus, Plato has Socrates relate the myth about Theuth. This myth thus appears as a kind of encore, an epilogue, an hors-d'oeuvre (literally: outside the work), a supplement. But what starts as a supplement, is found to be the most essential part of Phaedrus. It is an accusation against writing since writing would replace living memory for a mnemonic device. Plato presents writing as the sign of a sign. Speech remains in animate proximity, in the living presence of mneme. Writing, which imitates and reproduces living speech, goes one degree further. 'The boundary (between inside and outside, living and non-living) separates not only speech from writing but also memory as an unveiling (re-)producing a presence from re-memoration as the mere repetition of a monument' (Dissemination, p.108-9). The difference between mneme and hypomnesis. The problem starts where the mneme, instead of being present to itself, is supplanted by archives, lists, notes, tales, accounts, chronicles: memorials instead of memory. But, as Derrida indicates, the 'evil' slips in within the relation of memory to itself, in the general organization of the mnesic activity. Memory always needs signs in order to recall the non-present, with which it is necessarily in relation. The line between mneme and hypomnesis becomes barely perceptible because in both cases it is a matter of repetition. Memory is always already contaminated by its first substitute: hypomnesis. The outside (the replacing sign) is already within the work of memory. What Plato dreams of is the possibility of a memory with no sign; that is, with no supplement (cf. Dissemination, p.109).
Plato considers writing to be external to internal memory - hypomnesis is not in itself memory. Derrida, however, points out that Plato has to admit that writing or hypomnesis penetrates the very core of speech and mneme; it affects and infects memory. Writing is '... that dangerous supplement that breaks into the very thing that would have liked to do without it yet lets itself at once be breached, roughed up, fulfilled, and replaced, completed by the very trace through which the present increases itself in the act of disappearing' (Dissemination, p.110).

[2] In the myth, the god Theuth offers writing as a pharmakon to King Thamus of Egypt. It is a recipe for both memory and wisdom. Who is this Theuth, Plato's god of writing? Derrida shows that Plato's Theuth has much in common with two other gods of writing, the Egyptian god, Thoth and the Greek god, Hermes. In Egyptian mythology, Thoth often calls himself the son of the sun-god, Ammon-Ra. Ammon: 'the hidden'. The hidden sun, the father of all. He allows himself to be represented by Thoth. Thoth speaks in the name of Ammon-Ra. Thoth is the language through which Ammon-Ra enters the human world. In Derridian terms, this means that the father needs language in order to appear. Like his Greek counterpart, Hermes, Thoth is the messenger-god, an intermediary. But he can only convey what has already been thought by Ammon. Language is thus considered to be a representation of a more original thought. 'The message itself is not, but only represents, the absolutely creative moment. It is a second and secondary word' (Dissemination, p.88). Thoth, Hermes, and Theuth: the gods of writing are subordinate characters. They are but servants and executors. Never the authors or initiators of language.
But Thoth is also the substitute for Ammon-Ra, supplementing him and supplanting him in his absence. Thus, the written word of Thoth replaces the spoken word of the father. 'As a substitute capable of doubling for the father, the sun, and the spoken word, distinguished from these only by dint of representing, repeating, and masquerading, Thoth was naturally also capable of totally supplanting them and appropriating all their attributes' (Dissemination, p.90).
This brings into play a strange kind of logic. Thoth is opposed to its other - the father, the sun, speech, origin - but at the same time he is the only one capable of supplementing and supplanting it. Thoth extends or opposes by repeating and replacing. He distinguishes himself from his opposites, but also imitates them, replaces them by becoming their sign and representative. He is different than the sun and the same as the sun, different than the good (because he writes) and the same as the good (because he is the word of the father). The figure of Thoth takes shape and takes its shape from the very thing it resists and substitutes. But it thereby opposes itself, passes into its other. He is at once the father, the father's other and the subversive movement of replacement. This messenger-god, this god of non-identity, this supplement, is a god of the absolute and continous passage between opposites: an undecidable! (cf. Dissemination, p.92-3).