Just to clarify, Plato isn't really against writing per say — he did author all those Dialogues after all. He just doesn't like people confusing "knowing something" with "knowing where to find knowledge about something," e.g., looking it up in a book instead of having it in your memory. He says if people rely too much on books, they'll stop using their minds and will forget how to memorize information.

He's also afraid that people will learn subjects only from a book, rather than directly from a master in a teacher-student relationship.