...But all this well-laboured system of German antiquities is annihilated by a single fact, too well attested to admit of any doubt, and of too decisive a nature to leave room for any reply. The Germans, in the age of Tacitus [56-120AD], were unacquainted with the use of letters; and the use of letters is the principal circumstance that distinguishes a civilised people from a herd of savages incapable of knowledge and reflection. Without that artificial help, the human memory soon dissipates or corrupts the ideas intrusted to her charge; and the nobler faculties of mind, no longer supplied with models or with materials, gradually forget their powers; the judgment becomes feeble and lethargic, the imagination languid or irregular. Fully to apprehend this important truth, let us attempt, in an improved society, to calculate the immense distance between the man of learning and the illiterate peasant. The former, by reading and reflection, multiplies his own experience, and lives in distant ages and remote countries; whilst the latter, rooted to a single spot, and confined to a few years of existence, surpasses, but very little, his fellow-labourer the ox in the exercise of his mental faculties. The same, and even a greater difference will be found between nations than between individuals; and we may safely pronounce that, without some species of writing, no people has ever preserved the faithful annals of their history, ever made any considerable progress in the abstract sciences, or ever possessed, in any tolerable degree of perfection, the useful and agreeable arts of life....
Decline and Fall of the Roman Empire 
London: Everyman’s Library, 1993
Volume I, pp. 242-243