Derrida notes that critics have historically assumed that Plato flatly condemns writing in The Phaedrus. Derrida shows that writing, in Plato's text, is given a paradoxical quality of being useful and harmful (pharmakon), like a drug, a poison and a cure. Derrida does this by reinterpreting the myth of the origin of writing as described in The Phaedrus. 

In one of the first myths discussed, the Boreal wind caught Orithyia and flung her to the Illisus River. The river might be alluring because of supposed curative powers (cure) but she drowns (poison). Paralleling this myth, Socrates is drawn out of the city to the country, not by Phaedrus' verbal speech, but by a written text Phaedrus has. This establishes a parallel between a natural (wind) and man-made (text) phenomenon; both are seductive and both have good and bad potentials.

(It should be noted here that the reason writing was thought to be harmful at all is because speech has historically been considered more present, closer to the speaker's thoughts, his/her "Being," and thus closer to the truth of the speaker's living thought. Writing, not as "alive" as the speaker, had been thought of as "outside" the speaker and thus foreign to the purity of thought-spoken-truth. Writing, in this biased sense, is "repeating without knowing." In many of his works, Derrida deconstructs this supposed hierarchy of speech/writing.) 

In the first section, "Pharmacia," Derrida establishes the link between writing and myth (in opposition to speech as seemingly more original and therefore more true): 

The link between writing and myth becomes clearer, as does its opposition to knowledge, notably the knowledge one seeks in oneself, by oneself. 

In "The Father of Logos," the myth of Theuth is explained. Theuth is an Egyptian god who invented writing. Theuth presented writing to the king/king of the gods as a "recipe for both memory and wisdom." The usefulness of a recipe is uncertain because it is something that must be put into practice. It is as uncertain as a remedy or a drug that could be a poison or a cure. Socrates notes that unlike speech, writing is like a son without a father, no one there to attend to or defend it. Although Theuth presents writing as a cure, the king dismisses it as a poison. 

Without the presence of the father (or analogously the speaker), a written text can not represent pure, unchanging truth because it is open to interpretation. A present speaker on the other hand can defend his/her words in person. Writing is here established as being uncertain (again, like a pharmakon). Writing can supplement memory but it can hinder memory if one relies too much on it and not their internal thoughts. 

The suggestion is that writing is like myth; either can be good or bad. Obviously, the same argument can be made for speech. Derrida shows how this duality is not so much that myth and writing are paradoxical (poison/cure), but that Western thought has been dominated by binary oppositions: father/son, man/woman, speech/writing. And it is this structure of thinking that determines the way we think about things. Therefore, writing was forced into one of these binaries where the first term is given a position of higher authority. 

To structure meaning in these inflexible binaries ignores that each element (speech/writing) are capable of good or bad applications; neither is necessarily better. This binary structuring limits meaning. Deconstructing itself, the myth shows this.