Between the two periods of the revival of learning, the Italian and the Transalpine, a marked line is drawn by the invention of printing, _c._ 1455: when the one movement had run half its course, the other scarcely begun. The achievements of the press in the diffusion of knowledge are often extolled; and some of the resulting good and evil is not hard to see. But the paramount service rendered to learning by the printer's art was that it made possible a standard of critical accuracy which was so much higher than what was known before as to be almost a new creation. When books were manuscripts, laboriously written out one at a time, there could be no security of identity between original and copy; and even when a number of copies were made from the same original, there was a practical certainty that there would be no absolute uniformity among them. Mistakes were bound to occur; not always at the same point, but here in one manuscript, there in another. Or again, when two unrelated copies of the same book were brought together, there was an antecedent probability that examination would reveal differences: so that in general it was impossible to feel that a fellow-scholar working on the same author was using the same text. Even with writers of one's own day uniformity was hardly to be attained. Not uncommonly, as a mark of attention, an author revised manuscript copies of his works, which were to be presented to friends; and besides correcting the copyists' errors, might add or cut out or alter passages according to his later judgement. Subsequent copies would doubtless follow his revision, and then the process might be repeated; with the result that a reader could not tell to what stage in the evolution of a work the text before him might belong: whether it represented the earliest form of composition or the final form reached perhaps many years afterwards. To understand the conditions under which mediaeval scholars worked, it is of the utmost importance to realize this state of uncertainty and flux. Not that in manuscript days there was indifference to accuracy. Serious scholars and copyists laid great stress upon it. With insistent fervour they implored one another to be careful, and to collate what had been copied. But there are limits to human powers. Collation is a dull business; and unless done with minute attention, cannot be expected to yield perfect correctness. When a man has copied a work of any length, it is hard for him to collate it with the original slowly. Physically, of course, he easily might: but the spirit is weak, and, weary of the ground already traversed once, urges him to hurry forward, with the inevitable result.