Before the advent of printing in the middle of the fifteenth century, treatises in medieval science and natural philosophy depended for their existence on manuscript copies. As a consequence, they were subject to all the vagaries and uncertainties of any system that must rely on a scribe or copyist to produce one or more copies from an exemplar or to record a lecture as it was given. Medieval Latin texts were subject to more than the ordinary scribal vicissitudes — errors of commission and omission because medieval copyists had developed an elaborate system of abbreviations that served to speed the process of copying and also tended to save paper. These abbreviations frequently added an element of uncertainty to an interpretation of the text, both for someone who wished to read it as well as for someone who wished to copy it. The difficulties in deciphering medieval manuscripts affect modern understanding of medieval science in two basic ways.
The first way in which the difficulties in deciphering medieval manuscripts affects our understanding today concerns the integrity of an author’s work as it was copied and recopied and read by students and scholars over the course of centuries. Because copies might vary drastically as a result of scribal errors introduced at any point in the dissemination process, we may infer that the reader’s understanding of an author’s intent in some, and perhaps in many, passages was almost unavoidably distorted. Reliance on handwritten and handcopied works meant that versions of the same treatise in Paris, Oxford, and Vienna might differ substantially. In astronomical and mathematical texts, for example, essential diagrams and figures may have been included in some versions, but omitted or included only partially in others. Even when a diagram was included, scribal errors might reduce or destroy its utility. In purely verbal texts, words might be omitted or added by the scribe. Many of the copies of medieval works that have survived were not made by professional scribes, but by students who had copied the texts for their own use. Such copies were often passed along to other students, who would introduce more errors and changes. To these formidable problems, we must add that of legibility. The handwriting of copyists was frequently difficult to decipher and all too often was simply unintelligible.
University stationers, or booksellers, had as their responsibility the production of reliable texts for university personnel. They would often receive the pristine version of a treatise directly from its author. From this original they would make one or more copies. The stationers were authorized to lend all or parts of the texts to students who, for a fee, could copy it for their own use. Obviously, student copies varied in quality. Many were subsequently passed on to other students for further copying. Errors were inserted at virtually every stage of the process of multiplying and disseminating texts. Perhaps the only exception to this generalization are copies of the Bible, which were carefully supervised.
The second way that the interpretation of medieval manuscripts may affect our understanding of medieval science has to do with the limits imposed on modem scholars who read or edit treatises written in the Middle Ages. Most scholars would probably begin with a list of the extant manuscripts of the treatise in question. The quality of those manuscripts, which managed to survive the ravages of time, determines the level of intelligibility of that treatise. In most instances, significant gaps in our understanding of that treatise will probably remain even after modern scholars have completed an edition of it.
It is evident that differences between an original version of a medieval treatise and all the copies that were subsequently made from it were at best considerable and at worst vast. From our vantage point, we can see how difficult it must have been to do science in the Middle Ages. The preservation of reasonably faithful versions of the basic Greco-Arabic texts that had been translated into Latin was itself a major task. To this we must add the vast array of medieval scientific texts, commentaries, and questions that were copied and recopied. Unfortunately, not all texts were copied and recopied. Many treatises simply disappeared. During the Middle Ages, knowledge was as likely to vanish as to be preserved. An enormous effort would have been required just to maintain the status quo, or to restore a text that had been corrupted. Although we cannot measure the detrimental effects on medieval science and natural philosophy that followed solely because of a dependence on handwritten manuscripts, we may plausibly conjecture that they were enormous.
The introduction of printing in the mid-fifteenth century significantly altered this picture. With the advent of printed books, knowledge in general, and technical information in particular, could be disseminated with a speed and accuracy that could scarcely have been imagined in the age of manuscripts. Science was a particular beneficiary of printing. Identical copies of a scientific work could be spread through Europe in a relatively brief time. And yet, the precise role of printing in the generation of the Scientific Revolution is in dispute. We must ask if, in the absence of printing, the old scribal system could have been improved to multiply copies of scientific treatises and thereby meet Europe's intellectual needs. And would the ever-expanding royal, ducal, municipal, and university libraries have provided European scholars with sufficient access to allow the continued expansion of science and learning? Fortunately, we need not answer these questions in this study. The foundational contributions to early modem science that are its focus had already been formed long before Gutenberg’s printing press converted Europe from a manuscript to a print culture.
Although manuscript reproduction and dissemination posed serious problems in the Middle Ages, we must not conclude that the problems were insuperable. Despite the formidable obstacles just described, the quality of the handwritten texts available to medieval scholars in science and natural philosophy was often more than adequate to allow for their comprehension and for the addition of significant contributions to learning. The legacy that has reached us is one that we can comprehend and often admire. The core of that legacy was Aristotle's natural philosophy, which was deeply rooted in the medieval university […].
— Edward Grant, Foundations of Modern Science in the Middle Ages (New York: Cambridge University Press, 1996), pp. 51-53.