Among the friends of Johannes Kepler (1571–1630), the name of David Fabricius (1564–1617) stands out for two reasons. First, as an observational astronomer he initiated the study of variable stars. Second, he was an ardent devotee of astrology. In keeping with the latter enterprise, he collected horoscopes. On 18 July 1602 Kepler sent his own horoscope to Fabricius. Dissatisfied with the lack of detail in what he had received, Fabricius pressed Kepler for additional information. In particular he wanted to know “on what day in the carnival season in the year [15]91 a fever attacked” Kepler.

Kepler's reply is preserved only in a copy prepared by a hired scribe. This copyist did not always understand what Kepler had written in his draft of the letter. In some cases the copyist made mistakes; in other cases he simply omitted what he could not read. Thus, where Kepler answered Fabricius's question about the fever in 1591, the surviving copy says:

In the past few years a couple of unprecedented events have taken place in the normally rather quiet field of Bacon studies. Several hitherto unknown Bacon manuscripts, the most substantial ones to have come to light since the seventeenth century, have been identified. In addition to that, a whole new branch of Bacon's philosophy, the branch I have called the “speculative philosophy,” has been discovered and put together again. These two developments turn out to be mutually reinforcing. The new manuscripts tell us a lot about the speculative philosophy, and what we already know about the speculative philosophy from the printed sources helps us to make sense of the manuscript materials – materials that promise to give us new insights into the growth, scope, and character of the speculative philosophy itself.

Until recently it was generally believed that the canon of Bacon's work had been substantially established by the great Victorian editors Spedding, Ellis, and Heath. But in 1978 an unpublished natural-philosophical manuscript was found in the British Library (Additional Manuscripts 38,693, fols. 29r–52v). A transcription of and commentary on this piece was published in 1981. However, that discovery was quite overshadowed by findings made by Dr. Peter Beal in the course of his researches for the monumental Index of English Literary Manuscripts. Beal discovered a manuscript copy of an unknown fragment, Historia et inquisitio de animato et inanimato, a copy (possibly complete) of the Abecedarium novum naturae, and a 13,500-word Latin manuscript on biological topics.

It is my contention that the occult and the experimental scientific traditions can be differentiated in several ways: in terms of goals, methods, and assumptions. I do not maintain that they were exclusive opposites or that a Renaissance scientist's allegiance can be settled on an either/or, or yes/no, basis. Rather, in many instances, especially in the late sixteenth and seventeenth centuries, a spectrum of beliefs and attitudes can be distinguished, a continuum from, say, absolutely magical to absolutely mechanistic poles, along which thinkers place themselves at various points depending on their attitudes to certain key topics. One of these topics, not much discussed so far, is the relationship between language and reality. In the scientific tradition, I hold, a clear distinction is made between words and things and between literal and metaphorical language. The occult tradition does not recognize this distinction: Words are treated as if they are equivalent to things and can be substituted for them. Manipulate the one and you manipulate the other. Analogies, instead of being, as they are in the scientific tradition, explanatory devices subordinate to argument and proof, or heuristic tools to make models that can be tested, corrected, and abandoned if necessary, are, instead, modes of conceiving relationships in the universe that reify, rigidify, and ultimately come to dominate thought. One no longer uses analogies: One is used by them. They become the only way in which one can think or experience the world.

We now recognize that magic played a much greater role during the Renaissance than modern scholars at first were willing to recognize. Until recent decades, there was a tendency to think of magical thought as a kind of aberration in Western culture, one more appropriate to primitive societies than to the sophisticated European culture, and to feel that those magical ideas which did appear in the West should be referred to apologetically, if at all, as though admitting to a regrettable weakness. Even Lynn Thorndike's monumental History of Magic and the Experimental Sciences, although it gives evidence of a shift of attitude during the years it took him to write the work, for the most part speaks disparagingly about those who paid even lip service to magic. In the Introduction to the first volume Thorndike sounds apologetic about devoting so much effort to such fruitless ideas and justifies them in an antiquarian fashion. By the time of his last volume, however, he is willing to endorse Keynes's description of Newton as the last of the magicians and the first of the moderns. Indeed, in his discussion of Newton he seems almost to suggest that magic may have had some beneficial influences.

More recently, Renaissance magic has received a good deal of attention, spurred by such works as Frances Yates's Giordano Bruno and the Hermetic Tradition and D. P. Walker's Spiritual and Demonic Magic. As a result of this and other work, we now recognize that magic was an important element in Renaissance thought and cannot be ignored.

In a treatise on witchcraft first published in Trier in 1589 a German bishop explained that all apparently occult operations that were not in fact miracles could be ascribed in principle to physical causes. For whether or not any particular instance was actually demonic in inspiration, “magic” was simply the art of producing wonderful natural effects outside the usual course of things and above the common understanding of men. It followed that “if this part of philosophy was practised in the schools in the manner of the other ordinary sciences … it would lose the name of ‘magic’ and would be assigned to physics and natural science [et Physicae naturalique scientiae asscriberetur].” Likewise, in a set of theses on magical operations and witchcraft published a year later in Helmstädt, a natural philosopher and physician began by arguing that “magical actions and motions are reducible to considerations of physics [Ad Physicam considerationem reducuntur motus et actiones magicae]”.

The essays collected in this volume were originally given at a symposium that I organized in June 1982 at the Centre for Renaissance Studies of the ETH, Zürich. The Eidgenössische Technische Hochschule was founded in 1855 on the model of Napoleon's Ecole Polytechnique at Paris and in the wake of similar foundations at Berlin, Vienna, Munich, and Stuttgart, all of which were designed to supplement the arts curriculum of the older universities with teaching and research in science, technology, and architecture. Because the ETH has had a department of the humanities from its foundation (the first professor of art history was Jacob Burckhardt; Francesco de Sanctis held the first chair of Italian), and because its modern luminaries include both Einstein and Jung, it may be thought a not inappropriate setting for a conference on the relations between science and the occult. Whether or not the genius of the place exerted an influence on the proceedings is a question that had better be left open. At all events, the discussions were extremely lively and were marked by frequent challenging references to the texts (delegates seemingly happening to have with them copies of Thomas Aquinas, Newton, Cornelius Agrippa, and others). Among those who took a valuable part in the discussion, but who are not represented in this book, I should like to thank J. E. McGuire (University of Pittsburgh), Richard Gordon (University of East Anglia), G. A. J. Rogers (Keele University), and Keith Hutchison (University of Melbourne).

Whether for better or for worse, it is no longer possible for historians interested in the “scientific revolution” to regard the movement solely in terms of the victory of true and rational scientific ideas over the scholastic and magical modes of thought circulating in the sixteenth and seventeenth centuries. Not only have the attitudes of various men of science toward scholasticism and Aristotelianism been scrutinized, but the extent to which these men created a solely rational construction of reality has also been questioned. Scholars such as Cassirer, Garin, Kristeller, and Yates have redirected our attention to the importance of the “occult tradition” in generating and disseminating the new scientific modes of thought. Their claim is that Neoplatonism, hermeticism, astrology, alchemy, and the cabala – individually or as a unified ideology – had as great an influence on Kepler, Galileo, or Newton as they did on Ficino, Agrippa, and Bruno.

To be sure, not all historians of science share this perspective. Even those who accept the importance of the occult tradition vary in the degree of their commitment. Paolo Rossi, one of the earliest proponents of the occult tradition, has recently voiced certain reservations:

On the whole, Newton preferred not to publicize his involvement in alchemy. Unlike his other major pursuits, nothing of his alchemy, or at least nothing explicitly labeled as alchemy, appeared in print during his lifetime or in the years immediately following his death. A few people did know about it. A fascinating correspondence between Newton and John Locke following the death of Robert Boyle reveals that the three men, possibly the last three men from Restoration England whom one would have expected, only a generation ago, to find so engaged, exchanged alchemical secrets and pledged each other to silence. John Conduitt, the husband of Newton's niece, who gathered material about his life, knew of his experiments in Cambridge and reported that his furnace there remained an item of curiosity shown to visitors. Nevertheless, the adjective Conduitt used was “chymical,” not “alchymical,” and in a similar manner knowledge of Newton's interest in the art quickly sank from view. When David Brewster found alchemical manuscripts in Newton's own hand among his papers, he was appalled and quickly dismissed them as a curious relic of an earlier age. It waited until the twentieth century for the record to become public, with the auction of the papers still in the hands of the Portsmouth family, and for scholars to come to grips with it. Lord Keynes purchased some of the alchemical papers at the auction and insisted forcefully on their importance, but only in our own generation have scholars ready to take the papers seriously systematically studied the entire corpus, or rather that part – well over 90 percent – of the corpus known to exist that is available to the public.

In the Copernican description of the planetary system there are six planets instead of the Ptolemaic seven, the moon having become a subsidiary body of a type new to astronomers – and for which Kepler was to invent the term satellite in 1611. In 1540, Rheticus felt the need to defend this new number of the planets:

As Rosen remarks in his note on his translation of this passage, Rheticus's numerological argument finds no parallel in the work of Copernicus himself. It does, however, find an answer in Kepler's defense of Copernicanism in the Mysterium cosmographicum.

Kepler's own explanation of the number of the planets is geometrical: There are exactly six orbs because there are exactly five regular solids to define the spaces between them. As Kepler points out, the fact that there are exactly five such solids is proved in a scholium to the last proposition of Elements, Book XIII. Kepler had, however, considered the possibility of a numerical explanation of the structure of the planetary system, in connection with his earliest attempts to find a pattern in the ratios of the dimensions of the planetary orbs.

It is sometimes the case that historians of science neglect the vigorous humanistic tradition of science – Aristotelian physics and Galenic medicine – which is represented in the Renaissance by a bibliography many times greater than that of the experimental literature to which they direct their attention. Such neglect can disguise to modern readers the nature of the conceptual problems encountered to some degree by all Renaissance thinkers and can suppress differences perceived by them, even if not apparent to us today. Sixteenth-century scientific debates share a vocabulary, a mode of expression, and a conception of argumentation and genre: They are divided by issues in virtue of which a generation of thinkers formulated their individual conceptions of the world and its workings. This chapter is devoted to the study of one such debate which was widely known and often quoted: that which opposed Girolamo Cardano (1501–76) to Julius Caesar Scaliger (1484–1558). Of the two, Cardano has attracted more attention because his writings (and especially the De subtilitate) lie on the fringes of occult and experimental literature; Scaliger's answer to the De subtilitate belongs squarely to the humanistic tradition of science. Cardano explicitly rejects Aristotelianism as a synthetic explanation of the universe and thus is seen as forward looking; Scaliger represents, in the traditional view, that dead bough of the tree of knowledge usually labeled scholasticism, which is characterized by empty verbiage, obscurantism, and incongruity with the real and the natural.

The scholars who took part in this symposium addressed themselves to a topic that has been much discussed in the history of science in the past twenty years. The extent to which the two great realms of “magic” and “science” – to give them their traditional names – influenced each other during the Renaissance is a fascinating and exciting question. One can distinguish, perhaps, three main stages in its elaboration so far. In the first the history of science was seen as a narrative of progress through inventions and discoveries, an ever-improving movement toward positive knowledge. In this history of scientific triumphs, magic and the occult could be simply dismissed as entertaining but irrelevant. Even as late as 1957 Herbert Butterfield, in The Origins of Modern Science, felt no qualms about dismissing the occult tradition and its historiographers in the most sweeping terms. Van Helmont, we are told,