As always, in Plato there are more levels to this. Plato thinks that all true knowledge comes from memory, and that we gain knowledge by "remembering" things we

As always, in Plato there are more levels to this. Plato thinks that all true knowledge comes from memory, and that we gain knowledge by "remembering" things we first encountered before birth but have since forgotten. So it makes sense that he would see memory, rather than (even philosophical) writing, as the ultimate source of truth.