Astrological textbooks in different intellectual movements in the 16th century

Da die Dissertation inzwischen von meinen Betreuern angenommen und inzwischen auch verteidigt wurde, sei es mir gestattet, einige Ergebnisse der Arbeit vorzustellen. Es handelt sich hier um einen leicht gekürzten und überarbeiteten Vortrag, den ich im Warburg Institute (London) am 27. Februar 2014 gehalten habe:

This presentation is on the results of my dissertation. The title of the project is: “Astrology, a science of the 16. century? astrological textbooks and ephemerides in Latin at the area of conflict between scholasticism, humanism and reformation”.

This project has two aims. Firstly, I wish to create a complete list of astrological textbooks in Latin published until 1610. To give you a few numbers: I have collected 121 printings of ancient and medieval textbooks, 61 printings of contemporary textbooks, 65 ephemerides and many other sources which belong to 16. century astrology. Secondly, I wish to discuss the contemporary prefaces and commentaries in which it is explained what astrology is and how it should work. In this matter I also wish to discuss how astrology is different from theology or magic.

In these contemporary texts I found many topics from which I can give you a short overview of the questions discussed: What is the difference between Astrology and Astronomy? In which matter is astrology a science? What is the difference between general and particular influences and predictions? Is astrology a practical or a theoretical science?

The last question is a reference to Aristotle, Metaphysics epsilon, where he differs between practical, theoretical and productive sciences. The theoretical science also belongs to mathematics, physics and theology. With reference to this there was also a discussion about whether astrology is pure mathematics or also physics.

Further questions were also discussed. For example: Can I achieve insight into divine providence with astrology? Can astrological influences be characterized only as material influences, or do they also attain spiritual or intellectual activity? Are celestial influences dependent on demons?

All these questions were answered in different ways. I will discuss only a few examples. But before I start with the citations I would like to introduce you to my major thesis. After I having read all those sources I differ them into four groups: these are the Italian Aristotelians, the Thomists, the Lutheranists and the Neoplatonists. The first three groups depend mostly on the philosophy of science of Aristotle but they are different in their vocabulary. The fourth group is quite heterogeneous and much different to the Aristotelians because they postulate a spiritual or harmonic connection between the stars and the world. One of the results of my work however is that astrological textbooks were only written by Aristotelians; almost my whole work therefore discusses Aristotelism in the 16. century

Italian Aristotelism

One example of an Italian Aristotelian is Girolamo Salio. Salio wrote a preface for his collection of astrological textbooks which was published in 1493 (source). There is also a reprint in 1516. This collection contains the Tetrabiblos and the Centiloquium of Ptolemy with the commentary of Haly Abenrodoan and further texts from Messahalla, Omar Tiberiadis and Hermes. This print is very important because it is the only print which contains the commentary of Haly Abenrodoan.

Salio writes in the preface the following: “A science is a habit which is acquired by demonstrations”. This is a reference to Aristotle especially to the last chapter of the second book of the Posterior Analytics. Salio wishes to clarify in his preface that astrology should only be a theoretical science which is based on demonstrations. My aim is not to explain the entire logic behind demonstrations. However in Aristotle, demonstrations could be used in mathemathics and physics. If we discuss physics, we can understand demonstrations as a proven knowledge which was created by many observations over a long period of time. Referring to this Salio distinguishes between demonstrative astrology and judicial astrology; the first explores the causality between the sky and the earth with clear and true observations, whereas the second uses these demonstrations to make predictions and conjectures.

One further Aristotelian was Georg of Trebizond. Trebizond wrote a commentary to the Centiloquium of Pseudo-Ptolemy and created a new translation from the Greek. It was written in about 1470 but was firstly published in 1540. Now we know that the Centiloquium wasn’t really a work of Ptolemy but in 16. century they believed until 1554 that Ptolemy himself wrote this collection of aphorisms. As Cardano proved that there are many theories not known by Ptolemy in the Tetrabiblos, it can’t be his work.

In the first aphorism of the Centiloquium (source) Ptolemy says that someone who wishes to make an astrological prediction, is only allowed to make general predictions. It seems that this is a contradiction to the Tetrabiblos, in which Ptolemy deals very extensively with particular predictions. Particular predictions are here the contents of book III and IV, which contain natal astrology. In this way, one should be able to foresee future events of human life. For example, when will you die, do you get a successful job and so on. Trebizond therefore needs to explain in his commentary how particular predictions are possible.

The first solution is, as Trebizond says, that astrologers are only able to make conjectures (coniectura) when they want to make a particular prediction. But when it is only a conjecture it doesn’t belong to science, as long as science is certain knowledge and only deals with secure and true dispositions. Therefore particular predictions are not scientific but belong to prudence. Why is this Aristotelianism? Because this is a clear reference to the Nicomachean Ethics of Aristotle, Book VI, in which he also says that prudence has nothing to do with science and deals with particular actions.

The next author is Girolamo Cardano. Cardano wrote a very long and famous commentary to the Tetrabiblos of Ptolemy which was firstly published in 1554. In this commentary, Cardano gives a hierarchy of certainty of the astrological science of four degrees (source): The first is mathematical science which is more certain than natural science because it is based on demonstrations. Then follows the knowledge that the movements of the skies are eternal and unchangable but that the sublunar world is corruptive. In my opinion, this is a reference to the Aristotelian work about generation and corruption, and this explains the uncertainty of natural science. After that the observations of the movements of the stars follows. Last, there is the conjectural or comparative science. This also contains the astrological predictions. This has the lowest degree of certainty because it follows the other three issues. Cardano clearly has a weaker concept of science because he also speaks about conjectural science and not only about demonstrative science.

To conclude this, Italian Aristotelism: You can see that there was a big effort to explain astrology with the philosophy of science based on Aristotle in the 16. century.


Beside the Aristotelian impact on astrology, Thomism was a very important movement in the 16. century. In this time Thomas Aquinas rose to a big authority especially for astrology. Some Thomists wrote astrological textbooks with respect to the theological norms of Aquinas. Important Thomists were, for example, Lucio Bellanti, Pedro Ciruelo and Francesco Giuntini.

Here, I have a citation of Berardo Bongiovanni. He wrote a Brevis Summa of the Summa Theologiae of Thomas Aquinas which was first published in 1565 and contains a complete paraphrase of the text of Aquinas. A short paragraph was reprinted in a collection of pamphlets on astrology in 1580 (source). It contains his paraphrase of S. Th. II-II, q. 95, in which Thomas Aquinas explains allowed and forbidden forecasts. At the first three words: “Divinatio est peccatum”, you can see that all Thomists characterize every divination as a sin. Their argument is etymological: With Divination you will acquire insight into divine power. This doesn’t contain every prediction, however because if you see that something happens with necessity or many repetitions, it doesn’t belong to divination. In a following section Bongiovanni wanted to limit the astrological predictions on weather forecasting. But this wasn’t the opinion of most of the Thomists. They were quite liberal and also did natal astrology. They had a very restricted view on astrology because every celestial influence affects only the matter and the qualities of the matter. If you say that some special human event comes from the stars, in the Thomistic view you are only able to say that this follows indirectly from the mixture of the temperaments in the human body. In this way, the human soul can also be affected because there is a connection between the human body and its soul.

The approach of the Thomists to astrology is not very different to the Aristotelians, for example you would find references to Aristotle and Thomas Aquinas by Ciruelo or Bellanti as well.


The Lutherans are a different case because they didn’t want to refer to this medieval tradition. The main reference point for them was Philipp Melanchthon. He was a close friend of Martin Luther and headman of the university of Wittenberg. His aim was to found the Lutheran university. He taught many astrologers and gave his best scholars positions at other Lutheran universities. He also wrote some textbooks about physics or psychology in which he explains science in respect to the ancient world, especially to Aristotle. One of these important works is his Initia doctrinae physicae, which was published firstly in 1549 and was reprinted many times. In this work he describes the celestial influences as a part of physics. I wish to discuss another source here.

The first publication in which Melanchthon deals with the special issue about astrology was a letter to Garcaeus in 1537. In this text he writes that we have a threefold distinction of influences in humans (source). This threefold distinction “tria genera actionis in homines” was repeated later in some other letters of Melanchthon which were published as prefaces to some astrological textbooks. This distinction belongs to actions which doesn’t come from the person himself; it therefore doesn’t belong to human free will. The first class of influences are natural influences. This contains the temperaments of the human body, the qualities of the four elements and so on. It also contains celestial influences because they only affect the sublunar world in this way. The second class are divine influences from God and the angels, which only causes good. He describes it as “supra natura”. The third influences are influences from the devil or from demons. They only cause bad actions in the humans and they are “contra natura”.

We see that Melanchthon makes a clear distinction between natural and spiritual influences, so astrology doesn’t belong either to demons or to angels. There isn’t a big difference between Thomas Aquinas and Philipp Melanchthon because both make clear that there is no connection between divine or demonic influences and the stars. The stars affect the sublunar world only with natural or corporal influences. Many astrological textbooks were written or published within the Lutherans and follow these principles. The most famous are, for example, the textbooks of Johannes Schöner, Johann Garcaeus or Heinrich Rantzau.


Beside the Aristotelian tradition I shoud mention the Neoplationists. In my dissertation I don’t refer much to the Neoplatonic tradition in the 16. century because my research has shown that almost every astrological textbook was written without taking into account this Neoplatonic writings. But why is that?

To clear this question, my reference point is Marsilio Ficino who translated and commented on the works of Platon and Plotin. When the modern literature refers to Ficinos’ astrology they take into account a criticism of Ficino against astrology which was written in 1477 and edited by Kristeller. This criticism was only an early sketch and never published. Modern literature also refers to De vita libri tres in which Ficino explains the use of talismans and the spiritual influence of the stars. As far I can see, unfortunately nobody refers to much criticism of Ficino against astrology which he wrote in his commentary to Plotin. It was only mentioned by Kristeller in his book about Ficino.

For my case, the commentary to Plotin is very important because it was read by the contemporaries. I have found references by Gabriele Pirovano or Sixtus van Hemminga.

If you look at the first citation, you can see a big difference between Aristotelism and Platonism in 16. century: “A coelestibus non omnia fieri, sive enim inanimata sunt non facere, quae ad animam proprie pertinent, sive sint animata, divina videlicet anima, non facere mala” (source).

This means that the human soul receives its celestial influences not by matter or by the temperaments but that he receives it directly through the divine soul which can’t do anything evil. I don’t wish to explain the difficult philosophical speculations in Plotin. However with this argument, Ficino denies that the constitution of the human body or his future events can be foreseen within the change of his temperaments. Instead of this, the human mood and its constitution belongs to the spiritual and intellectual forces of the sky and the signs.

In the second citation I wish to show you that in this kind of Neoplatonism, everything is connected in cosmic harmony like cosmic music (source). Here you can see how a bad mood in a person can come from the heavens. This doesn’t refer to good or bad aspects, as Ptolemy writes, but rather refers to the music of the signs which creates dissonance in a single person.

With further arguments Ficino denies the system of the houses and bad influences from the stars. You can imagine why Ficino only refers to the talismans in his famous work De vita libri tres. I think that in the commentary to Plotin and in De vita libri tres Ficino argues in a very similar way.

This argumentation was a big problem for the theologians, however, on the one hand, we have spiritual connections between the sky and the sublunary world. In this way this could be used to invoke demons which was forbidden in this time. On the other hand, spiritual or intellectual influences may affect human will. But free will was necessary for many theological doctrines.


Let’s make a conclusion. These four groups dealt with the influences of the stars in a special way. The Arisototelians, the Thomists and the Lutherans refer only to a natural causality which has nothing to do with spiritual or demonic influences. In contrast Platonists say that spiritual or demonic influences are much more important than these influences from the stars into matter. According to Ficino, most Platonists deny traditional astrology.

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