Socrates would have gone unknown were it not for the work of Plato. Seneca might have been a footnote in history had it not been for his work. The same could be said of Epictetus.
In each of these cases, there is but a single reason they occupy our thoughts today: they were enshrined by the written word. The written word transcends ordinary activity; it can preserve for the mind the long gone. It can outlast the greatest of the stone monuments, and it can come closer than any elixir to bestowing immortality.