An idea may reach another person or groups orally. But while it is may be airborne, its scope is pretty limited. A speech may infect an audience, a harangue may embolden an army, a sermon may excite a congregation, but fertilisation happens there and then. Classical rhetoric described the conditions of persuasion as decorum – the fitness of a persuasive act to its environment, the audience, the timing, in other words, the context.
The way to overcome the limitation of here and now is through writing. Writing breaks the immediacy, creating the conditions for epidemics of ideas. This in turn leads to mass communication, which evolves non-scriptural types of diffusion, like audio recordings and videos.
But back to writing. Demosthenes, Aeschines and Cicero may have been the most brilliant speakers of their times, but without the benefit of the written record, their ideas and rhetorical feats would have died with the audiences they moved and inspired. More than a thousand years later, we are still moved by their brilliance (if you’re not, go grab a Penguin). Half a millennium earlier, they shaped entire generations and provided the humanist impetus which ushered in the modern world.
Writing is neither here nor now. It can be everywhere and has the power to avoid natural death. Besides, script has a much higher lifespan than sound, which needs constant resurrection on other people’s lips to survive. Soundbites need script to become memes.
Written ideas generate epidemics on a much larger scale than spoken ideas and are not restricted in time. They may lay dormant for generations and then jump up to life.