Given the complexity of Derrida's essay "Plato's Pharmacy," eNotes space-limited tutoring format can give only a narrow glimpse at all that Derrida discusses in this essay that begins by attacking the Structuralist position that structure of texts can be known.
A text is not a text unless it hides ... from the first glance, the law of its composition and the rules of its game.
To begin with, Derrida's essay is a response to Plato's dialog, Phaedrus. The essay attempts to show that Plato establishes binaries that subvert or undermine his own message about subjects like the ascendancy of speech over writing and the possibility of clear and distinct use of words like the Greek word pharmakon, which has the dual meaning of "cure" and "poison."
One of the points Derrida makes in response to Roland Barthes's ideas is that reading cannot be a form of writing since the binary dismantles the logic of that notion causing reading and writing to "rip apart" into separate acts.
if reading is writing, this oneness designates neither undifferentiated fusion nor identity at perfect rest; the is that couples reading with writing must rip apart.
One of Derrida's objectives in this early essay is to demonstrate the deconstructionist principle that binary pairs undermine their messages through the Saussurean idea that it is the difference between signifiers that determines meaning--it is not the "definition" of the signifier that determines meaning because signifiers are only defined by what they are not. Thus one thing Derrida argues and seeks to demonstrate is that Plato's very logic and his rhetoric dismantle his own message about speech's primacy over writing.
The incompatibility between the written and the true is clearly announced at the moment Socrates starts to recount the way in which men are carried out of themselves ...