11. Conclusions

From this there emerges a picture quite different than that of an ingenious empiricist working in an intellectual vacuum. The evidence of Leonardo's notebooks confirmed that he was widely read and had many contacts. His extant treatises revealed much more structure than has generally been assumed. Moreover, they evidenced a number of clear plans for books. Examination of his entire extant corpus brought to light another unexpected feature: for all their universality the notebooks are focussed on a surprisingly small number of basic themes: crucial among these are his studies of transformational geometry and a mechanical approach to nature, which uses as a point of departure his concept of four powers (weight, force, motion and percussion), and serves ultimately to integrate both his study of the microcosm (anatomy) and the macrocosm (astronomy) within a single grand plan. It was shown that these studies were guided by a distinct method of listing variables systematically and playing with them experimentally. It was claimed that the results of this enterprise inspired him to write treatises and led him to make serious plans for publication. While these plans were unsuccessful, there is nevertheless evidence, mainly indirect, that Leonardo, who was at the centre of action in the major cities of the high Renaissance (notably Florence, Milan and Rome), was not without influence in the century that followed. To explain why Leonardo was subsequently forgotten, the limitations of his approach were considered, as were the limitations of historiography in our own century.

All this was guided by two further purposes: a) to establish that Leonardo and the sixteenth century context leading to Galileo require more careful study, and b) to suggest that new combinations of earlier approaches are required. It is possible, for instance, to accept that there was some continuity between the thirteenth and the seventeenth centuries without assuming that nothing happened between the time of Grosseteste and Galileo. If Leonardo's knowledge of mediaeval and classical sources was not exhaustive, it was almost certainly for a reason. He decided that they could not provide him with an adequate picture. So he studied these sources and challenged them also. This process continued into the seventeenth century. In 1625, for example, Accolti^[405], whom one might have expected to cite Kepler, still acknowledged the thirteenth century writer, Witelo, as the most important authority on optics. At the same time Accolti treated Witelo critically and challenged him. The continuity question offers a means of documenting how specific experiences and experiments were gradually seen as legitimate tools to challenge first individual passages and finally the very authority of traditional sources. This is of interest, but in our view there are more pressing issues.

While accepting some continuity we have suggested that there was a breakthrough in science during the period 1490-1510. Interestingly enough this period coincides with the high renaissance in art and thus confirms in an unexpected way Cassirer's claims concerning links between science and art. But whereas he focussed on philosophical context, we have shown that the practical context of machines and instruments played a central role. That the history of technology offers a key to understanding developments in the history of science is perhaps the most important issue raised by Leonardo's work. It means that marxists such as Zilsel were right about the importance of craftsmen. It also means that Zilsel's claims need revision on two counts. First, Leonardo's life confirms that there was no invisible social barrier preventing different classes from meeting prior to 1600. In theory, as an illegitimate son Leonardo had little social standing, yet he was as a brother with Jacopo Andrea da Ferrara^[406], a leading Vitruvian commentator, he was friends with dukes, popes and the King of France. Secondly, as we have shown, Leonardo combined both of Zilsel's ingredients of science, causal thinking plus systematic organization, in the period 1490-1510, over a century before Gilbert, Galileo and Bacon. Indeed, as we have noted, Leonardo was engaged in writing this in the form of a treatise in his Elements of machines and was close enough to completion that Pacioli could mention it in print in 1509. Hence, whereas Zilsel and everyone since has assumed that the sixteenth century could provide only isolated examples of causal thinking, we now have firm evidence of both causal thinking and systematic method in the first decade of the century. In the midst of the high renaissance it is highly unlikely that insights of such magnitude would simply be forgotten. So we need to look afresh, and much more closely^[407] at the sixteenth century if we are to see in a proper framework the contributions of Tartaglia, Benedetti, Guidobaldo del Monte, Galileo and later thinkers; to establish a chronology of when, which discovery or invention was made where, in order that individual scientists and craftsmen are given due credit.

It is important to recognize that Leonardo's example also raises deeper questions of method. For it suggests that questions of why and how the scientific revolution took place are not simply abstract problems of philosophy, psychology or sociology, but intimately connected with the historical evidence itself. If Leonardo had only an astrolabe, a quadrant and a few isolated gadgets, he could not have dared to make his claims about machines in universal terms. The great number of machines and instruments with which he dealt was a vital ingredient in making the universality of his claims possible and credible. Hence, aside from individual characteristics of given machines and instruments, upon which historians of technology have traditionally focussed their attention, there is a cumulative dimension to their development which makes science possible and which requires study. Needed is a history of how, what were originally seen as a great variety of individual procedures, techniques and methods, were gradually recognized as part of a single, cumulative programme of mathematical sciences, how trigonometry gradually became an integrating tool. This goes beyond Strong's arguments about procedures and requires more than internalism. Thus far we have encyclopaedias of techniques in terms of first occurrences and latest developments. We need a record of what came in between if we are to make some map of this cumulative interplay between technology and science that is unique to the West. This is not to deny the significance of specific references in the manuscripts and printed texts of the period to Archimedianism, Platonism, Pythogoreanism, Vitruvianism, hermeticism, mysticism and other metaphysical influences. Nor is it to question the value of philosophy, psychology and sociology as tools for the history of science. What the discovery of structure and method in Leonardo's notebooks suggests rather, is that we have not looked closely enough even at the basic facts and that a deeper understanding of the history of science will require that we begin focussing on its history.

In the 1490's an important development in the use and study of machines and instruments provided one of the key shifts that made possible what is now remembered as the scientific revolution. It was accompanied and partly inspired by a fresh examination of historical sources which printing had recently made newly accessible. In the 1990's an analogous mechanical revolution may be occuring in the spread of electronic devices. The latest developments in computer memory make feasible for the first time a history of scientific instruments and techniques which will include not only the first and the last but also stages in between.

Paradoxically, without the help of computers and related modern instruments (CD-ROM, videos, etc.) a systematic history of these developments in instrumentation connected with Leonardo is not feasible; the enormity of the evidence connected with the mechanical-instrumental revolution of the 1490's is only becoming possible with the new mechanical means of the 1990's. Perhaps our new scientific revolution will again be accompanied by a parallel revolution in access to historical knowledge, and possibly this more systematic approach to the past will lead to a new appreciation of Leonardo and his method.