In the comments Giulio asked:
Will you ever (or have you already) write a detailed account on how astrology and more in general the various pseudo-sciences have been abandoned as scientific belief?
I have in the past actually blogged on this topic, but it was long ago, so I’ve decided to write an updated post about it. To some extent the topic is much too complex and too vast to deal with completely in one blog post, so this will be at best a sketch. The first thing is to define what exactly we mean by the pseudo-sciences or as I prefer to call them the occult sciences, where here the term science is used in its original, more general sense of knowledge, rather than any modern definition of the term and occult means hidden. The occult sciences fall into three general areas astrology, alchemy, and magic. Each of them ceased to be regarded as part of the accepted canon of scientific belief, in the modern sense, differently, and so must be discussed separately.
Today, when people use the term astrology, they generally mean natal or genethliacal astrology that is the casting of a horoscope or natal chart that includes the exact date, time, and location of a person’s birth. The interpretation of this chart supposedly maps out the subject’s life. However, traditionally this was just one branch of judicial astrology. The other branches of judicial astrology are mundane or political astrology, which deals with world affairs, electional astrology, which concerns the determination of the most auspicious moment to begin an undertaking or enterprise, which can be anything from a wedding to a war, and horary astrology that attempts to answer a question asked at the exact time the horoscope is cast. As well as judicial astrology there was natural astrology, which had two branches, astro-medicine or iatromathematics, and astro-meteorology, weather forecasting by horoscope.
Throughout the Middle Ages, various scholars rejected the legitimacy of judicial astrology, whilst at the same time accepting that of natural astrology, so I will deal with them separately, starting with the two branches of natural astrology. Astro-medicine or iatromathematics was, with one small exception which I will deal with later, the only area of the occult sciences taught extensively in the European universities. Astro-meteorology was also studied rather than taught at the universities, as a subdiscipline of astronomy. At the beginning of the seventeenth century iatromathematics was still taught widely and in fact had been the major reason for the establishment of chairs for mathematics at the Renaissance universities. However, as the century progressed it just sort of melted away as the innovations in medicine introduced during the Renaissance took over the curriculum. One cannot say there was a specific point where iatromathematics was formally rejected at the universities but by the end of the century it was no longer taught. However, outside of the universities it continued to flourish for quite a long time with both qualified physicians and other healers offering their astrological services for money to all those prepared to pay.
The case with astro-meteorology is different and presents an interesting case of the application of an empirical methodology. Already in the sixteenth century at the height of popularity of astrology, scholars were very much aware of the fact that the empirical basis or justification of astrology was, to say the least, very thin and some of them set about trying to correct this deficit. One area in which they did this was astro-meteorology. Already in the High Middle Ages people began to keep weather diaries in which they recorded the horoscope for the day alongside observations of the actual weather in order to determine if there was an actual correlation between the two. Roger Bacon (c.1220–c. 1292), who was a fervent believer in astrology, kept one, as did, later, Pico della Mirandola (1463–1494), who wrote the most infamous refutation of astrology. During the Renaissance weather diaries were kept by Johannes Stöfler (1452–1531), definitely pro astrology, the Nürnberg mathematicus Johannes Werner (1462– 1522), also pro, and the astronomers Tycho Brahe (1546–1522), David Fabricius ((1564–1617), and Johannes Kepler (1571–1630), all of them practicing astrologers. They, of course, found no correlation and astro-meteorology quietly died away during the seventeenth century. The weather diaries, however, laid the foundations for scientific meteorology.
The case of judicial astrology is somewhat different, apart from anything else it never found entry to the European universities. Having said that, nearly all professional astronomers in the High Middle Ages and the Renaissance were practicing astrologers and no self-respecting ruler was without an official court astrologer. This was still very much the case in the early seventeenth century but by the end of the century judicial astrology had also lost its status. Over time, various explanations have been offered for this decline. Earlier scholars, who didn’t actually study the history of astrology, attributed this decline to Pico’s Disputationes adversus astrologiam divinatricem published posthumously. However, this work was strongly rejected and refuted by astrologers and actually had very little impact. Another false thesis was that the advent of heliocentric astronomer dealt the death blow to astrology. However, the historian of astronomy Robert Westman famously stated in a footnote that there were only ten Copernican’s, those who accepted his whole system cosmology and all, between the publication of De revolutionibus in 1543 and 1600, a historian of astrology pointed out that all ten were practicing astrologers. The advent of heliocentrism had almost no impact on the belief in judicial astrology and why should it? Astrology is about the position of the planets on the ecliptic at a given point in time, the horoscope, which translates as time picture, and the ecliptic still existed in heliocentric astronomy.
As with astro-meteorology, there were attempts to provide natal astrology with an empirical basis in that collections of natal horoscopes were coupled with the biographies of the subjects, whose horoscopes they were. Johannes Petreius (c. 1497–1550), who published De revolutionibus, also published the first such printed collection, Libelli duo: De Supplemento Almanach; De Restitutione temporum et motuum coelestium; Item Geniturae LXVII insignes casibus et fortuna, cum expositione, by Girolamo Cardano (1501–1576) and an expanded edition, Libelli quinque, four years later. Petreius also planned to publish the much larger collection of Erasmus Reinhold (1511-1553), but he died before he could and the collection, which is still extant, was never published. Perhaps the most famous such collection is the Brief Lives of John Aubrey (1626–1697), which were only published for the first time in the nineteenth. As with the weather diaries, the horoscope collections failed in their aim to provide an empirical basis for natal astrology but did provide an impetus for the modern concept of history.
So why did natal astrology lose its intellectual status towards the end of the seventeenth century? Natal astrology, in fact astrology in general, took its metaphysical legitimisation, especially during its heyday in the Renaissance, from the microcosm-macrocosm analogy, as above so below, which had its roots in Greek philosophy. With the advent of the new modern metaphysics of the scientific revolution, astrology lost this legitimisation. It didn’t go away but was no longer considered academically respectable. The death knell was rung by Jonathan Swift (1667–1745) in his scurrilous Isaac Bickerstaff parodies in which he mercilessly attacked John Partridge (1644–c. 1714) a leading English astrologer of the period.
Alchemy is a completely different story as, unlike astrology, it was never established on the European universities. In fact, whereas astrology enjoyed for a long time almost universal acceptance, alchemy was in general viewed with widespread scepticism. Strangely, it was only through the work of the highly controversial Swiss physician Paracelsus (1493–1541) that alchemy became for a brief time partially acceptable. Paracelsus’ medical theories were heavily alchemical. He regarded the human body as a sort of alchemical furnace and many of his remedies were based on minerals rather the more conventional herbal cures. Because of this he is regarded as the founder of pharmacology. During his lifetime he was generally regarded as a figure of derision but following his death there developed a strong Paracelsian medical movement in the second half of the sixteenth century promoting his mineral based alchemical medicine, known as iatrochemistry.
The University of Marburg claims to have the world’s first chair for chemistry, established in 1609, with Johannes Hartmann (1568–1631) as its first incumbent. The chair was established by Moritz von Hessen Kassel (1572–1632) son of the astronomer Wilhelm IV. Unlike his father, Moritz, was a passionate supporter of alchemy and Paracelsian medicine. He founded the Collegium Chymicum and called Hartmann as Professor for Chymiatrie (iatrochemistry). This marks the beginning of a gradual transition or mutation from classical alchemy to modern chemistry. Because of its intermediary nature, being neither the one nor the other, historians of alchemy have taken to referring to it, during this period, as chymistry. Throughout the seventeenth century proponents of iatrochemistry included the Wittenberg professor of medicine Daniel Sennert (1572–1637), who played a central role in the introduction of corpuscula theory, schoolteacher Andreas Libavius (c. 1550–1616), who wrote a textbook on the topic, Alchemia, published in 1596, and the Belgian physician Jan Baptist van Helmont (1580–1644). Although all of them propagated iatrochemistry, they rejected Paracelsus, the man who had founded it, because of his mysticism.
At the end of the seventeenth century alchemy was still alive and well with Robert Boyle (1627–1691), who is normally credited with being the founder of modern chemistry, because of his The Sceptical Chymist (1661), Isaac Newton (1642–1726), and John Locke (1632–1704) all active practitioners, but they were amongst the last academic to be so. However, towards the end of the seventeenth and beginning of the eighteenth centuries, the German physicians Johann Joachim Becher (1635–1682), a fan of Boyle’s work, and Georg Ernst Stahl (1659–1734) developed the phlogiston theory out of Paracelsus’ matter theory. The theory was wonderfully wrong, but it was the first comprehensive scientific theory in chemistry and during the eighteenth-century various scholars developed a lot of chemistry out of it. During the second half of the century chemistry moved out of the medical faculty and became a scientific discipline in its own right. Alchemy was no longer considered respectable by academics. Interestingly, the early chemists, although they used the laboratory equipment developed by the alchemists and also their methods of analysing chemical compounds, pretended that their discipline had never had anything to do with disreputable alchemy.
Magic is a bit of a problem because it’s not one thing, but a collection of diverse practices gathered together under a single term. Although the existence of magic was accepted by almost everybody during the Middle Ages, it was, unlike astrology and alchemy, never considered acceptable by anybody, but was in fact condemned as demonic. This changed for a brief period during the Renaissance with the advent of so-called Natural Magic. This was the metaphysical belief that it was possible to directly manipulate nature by harnessing the celestial forces that governed astrology and alchemy. I’ve written a whole blog post about its genesis, so I’m not going to repeat it all here. Natural magic was, it seems a short-lived phenomenon, and the last scholar of note who investigated it was Francis Bacon (1561–1626), who read and extensively excerpted the vast Magia Naturalis of Giambattista della Porta (1535(?)–1615), but never published anything on it in his lifetime.
We have now reached the end of our trail and I think it is clear that the occult sciences were not so much killed off by the evolution of modern science in the seventeenth and eighteenth centuries but gradually faded into the background during that evolution. Although they ceased to be academically respectable, they never actually went away. If you google any of the terms’ astrology, alchemy, or natural magic today, you’ll find literally thousands of offers to initiate you into their secrets. In fact, in the last couple of weeks I’ve read serious magazine article in both English and German commenting on what they see as a serious renaissance of interest in astrology. What goes around comes around.