Bartosz Kaluziński: The debate on Galileo. Was the telescope a turning-point in the Copernican Revolution?

Uniwersytet im. Adama Mickiewicza w Poznaniu


Artykuł jest zestawieniem poglądów dwóch wielkich filozofów i historyków nauki – Paula Feyerabenda oraz Thomasa Kuhna. Porównane zostały stanowiska obu tych autorów dotyczące roli jaką odegrał Galileusz w trakcie przejścia od systemu Ptolemeusza do systemu Kopernika. Kluczową kwestią okazuje się być wiarygodność danych empirycznych zebranych przy użyciu teleskopu. Problem ten jest znacznie lepiej opisywany przez Feyerabenda. Jego anarchistyczne podejście pozwala na zupełnie nowe, odmienne od potocznego, spojrzenie na rolę zarówno Galileusza jak i Kościoła Katolickiego w trakcie Rewolucji Kopernikańskiej.


This article is a comparison of views of two great philosophers and historians of science – Paul Feyerabend and Thomas Kuhn. Both these authors were compared in respect to their ideas concerning the role played by Galileo during the transition from the Ptolemaic to the Copernican system. The key issue appears to be the reliability of empirical data gathered using a telescope. This problem is much better described by Feyerabend. His anarchist approach allows us to look at the role of both Galileo and the Catholic Church during the Copernican Revolution in a completely new and extraordinary way.


Galileo used a telescope to observe the sky to confirm the heliocentric[1] model of the universe, as proposed by Nicolas Copernicus in his famous opus De revolutionibus orbium coelestium (eng. On the Revolutions of the Heavenly Spheres), published in 1543 in Nurenberg, Gemany. Galileo was condemned by the Catholic Church for popularising Copernicus’ theory and his participation in the debate concerning the model of the universe is controversial – some people consider him a victim of obscurantism and intolerance whereas others see him as a brawler who violated the established social order. In 2008 we witnessed a student protest against the visit of Pope Benedict XVI at the Sapienza University of Rome, which was caused by the Catholic Church’s condemnation of Galileo. I will make an attempt to resolve the issue by evaluating the activities of both Galileo and the Catholic Church by studying the works of two outstanding philosophers and historians of science – Thomas Samuel Kuhn and Paul Karl Feyerabend. I believe that it is crucial to investigate the arguments presented by Galileo, and only then can we issue a final judgment concerning his activities. Simply speaking, assessing the actions of both the Catholic Church and Galileo depends on the methodological status of their arguments. A reliable assessment requires us to take into account the historical context, which means that we have to take into account the state of knowledge at that time and cannot judge the actors of past events by our own, contemporary measures. This is the reason for which our considerations are based on the works of Kuhn and Feyerabend – both of whom are historical relativists.

Introduction to Feyerabend’s anarchistic methodology

Feyerabend states that it is possible to develop science in a counter-inductive[2] way. Such a procedure rests on two pillars:

  1. the formulation and development of hypotheses contrary to the currently well-confirmed scientific theories
  2. the formulation and development of hypotheses contrary to the currently well-confirmed scientific facts[3].

Why does he make such statements? What is the explanation for the creation of theories contrary to the currently well-confirmed theories? Would it not be better to develop existing theories, because they are, as was said, well-confirmed? Feyerabend’s thesis calls for theoretical pluralism, which is always better than theoretical monism. It is related to objection to two methodological requirements that were formulated, respectively, in the tradition of logical positivism and Popper’s hypothetic epistemology:

  1. The consistency condition “which demands that new hypotheses agree with accepted theories is unreasonable because it preserves the older theory, and not the better theory. Hypotheses contradicting well-confirmed theories give us evidence that cannot be obtained in any other way. Proliferation of theories is beneficial for science, while uniformity impairs its critical power”[4].
  2. The autonomy principle which states that “facts exist, and are available independently of whether or not one considers alternatives to the theory to be tested. (…) It is not asserted by this principle that the discovery and description of facts is independent of all theorizing. But it is asserted that the facts which belong to the empirical content of some theory are available whether or not one considers alternatives to this theory”[5].

The autonomy principle is based on a totally incorrect presupposition which states that it is possible to test empirically scientific hypothesis by a simple confrontation of that hypothesis and the reality/facts that are directly accessible. But this is not a true account of hypothesis empirical verification and we need a more complex specification. Facts and theories are much more related than we commonly think:

“Not only are facts and theories in constant disharmony, they are never as neatly separated as everyone makes them out to be. Methodological rules speak of ‘theories’, ‘observations’ and ‘experimental results’ as if these were well-defined objects whose properties are easy to evaluate and which are understood in the same way by all scientists”[6].

According to Feyerabend’ account there are facts that are inaccessible without considering some alternative theory, thus we shall reformulate our idea of empirical verification:

“that the methodological unit to which we must refer when discussing questions of test and empirical content is constituted by a whole set of partly overlapping, factually adequate, but mutually inconsistent theories”[7].

All theories have some positive value, but this fact does not imply that all theories are equally good at explaining our reality. This means that in order to compare scientific theories, we must allow the possibility of their mutual coexistence, sometimes for a long time, even if these theories are contradictory:

“it is the normal case: theories become clear and ‘reasonable’ only after incoherent parts of them have been used for a long time. Such unreasonable, nonsensical, unmethodical foreplay thus turns out to be an unavoidable precondition of clarity and of empirical success”[8].

All standards of rationality and reasoning are determined by upbringing and education, which we perceive during the course of our life. Arguments convince us because of their frequency of repetition, not due to their semantic content[9]. Our problem is that, in most cases, we do not want to regard as obtained things that we consider as “standards” and “rationality”.

It is extremely important to know that Feyerabend explicitly notes that his intention is not to create a new set of methodological rules, compliance with which will lead us to the truth understood in the classical way as a correspondence between sentences (or theories) and reality. Feyerabend does not want to replace Karl Popper as the one telling scientists what their daily research practice should look like. His main idea is that every rule, or set of methodological rules, has limitations, no matter how rational it appears or how reputable a tradition it has, and these limitations have bad consequences – they block the development of science. Thus, there is only one methodology which does not negatively affect the development of science – and this is anarchistic methodology. The only principle which promotes scientific progress in all circumstances and does not block it is the principle of “anything goes”.

In the whole history of science there was no such methodological rule that was not broken, usually unintentionally. But this fact should not be a reason to blame the scientists, as it frequently brought some innovatory idea and revolutionised science. Breaking the rules could bring development, and strict adherence to rules could cause numbness and stagnation.

As we can see, the whole idea is based on the notion of scientific progress, but how can it be defined? What is true progress in science? Feyerabend’s answer is stunning:

“Incidentally, it should be pointed out that my frequent use of such words as ‘progress’, ‘advance’, ‘improvement’, etc., does not mean that I claim to possess special knowledge about what is good and what is bad in the sciences and that I want to impose this knowledge upon my readers. Everyone can read the terms in his own way and in accordance with the tradition to which he belongs. Thus for an empiricist, ‘progress’ will mean transition to a theory that provides direct empirical tests for most of its basic assumptions. Some people believe the quantum theory to be a theory of this kind. For others, ‘progress’ may mean unification and harmony, perhaps even at the expense of empirical adequacy. This is how Einstein viewed the general theory of relativity. And my thesis is that anarchism helps to achieve progress in any one of the senses one cares to choose”[10].

So, the most important features of methodological anarchism are:

1)      formulation and development of hypotheses contrary to the currently well-confirmed scientific facts and theories

2)      rejection of the consistency condition

3)      rejection of the autonomy principle

4)      the statement that theoretical pluralism is always better than theoretical monism – sustaining many theories is beneficial for the development of science

5)      the statement that rigid adherence to methodological rules in some situations may block progress.

Kuhn regarding Galileo

According to Kuhn, Galileo’s use of the telescope was an event of great importance:

“In 1609 the telescope was a new instrument, though it is not clear how new it was. Galileo heard that some Dutch lens grinder had combined two lenses in a way that magnified distant object; he tried various combinations himself and quickly produced a low-power telescope of his own. Then he did something which, apparently, no one had done before: he directed his glass to the heavens and the result was astounding. Every observation disclosed new and unexpected objects in the sky. Even when the telescope was directed to familiar celestial objects, the sun, moon, and planets, remarkable aspects of these old friends were discovered. Galileo, who had been a Copernican for some years before he knew of the telescope, managed to turn each new discovery into an argument for Copernicanism”[11].

What did Galileo notice when he observed the sky through a telescope? He saw the phases of Venus, many new stars, many craters on the Moon, and of course the four satellites of Jupiter. But was this the crucial argument against Ptolemy’s model of the universe? Did observations through this telescope provide conclusive empirical evidence? Kuhn’s answer is negative:

“The evidence provided by Galileo’s telescope is forceful, but it is also strange. None of the observations discussed above, except perhaps the last [the phases of Venus – B. K.], provides direct evidence for the main tenets of Copernicus’ theory – the central position of the sun or the motion of the planets above it. Either the Ptolemaic or the Tychonic universe contains enough space for newly discovered stars; either can be modified to allow the imperfection in the heavens and for satellites attached to celestial bodies; the Tychonic system, at least,  provides as good an explanation as the Copernican for the observed phases of and distance to Venus. Therefore, the telescope did not prove the validity of Copernicus’ conceptual scheme. But it did provide an immensely effective weapon for the battle. It was not proof, but it was  propaganda[12]”.

This passage allows us to understand how Thomas Kuhn understands the role of the telescope in the process of the Copernican Revolution, especially the role of empirical data provided by the telescope. Kuhn believes that every man was able to look through the telescope and to see that the universe looked different than it seemed. But it does not prejudge the truth of the Copernican model, because the Ptolemaic system is able to assimilate almost all refuting cases, except the phases of Venus. That case was well explained by the system created by the Danish astronomer Tycho Brahe. His theory was something of a mix between both the Ptolemaic and Copernican systems: Earth is central and stationary and rotates around its axis; there are two celestial bodies circulating around her, the sun and the moon; the planets (Mercury, Venus, Mars, Jupiter and Saturn) circle around the sun. Hence, the observations performed with the telescope did not bring empirical data which could easily falsify the Ptolemaic or, especially, the Tychonic system. It only changed many people’s state of mind; their belief in the Ptolemaic and Tychonic systems was undermined.

Thus, the role of the telescope was crucial, i.e., it did not bring proofs for the Copernican system but it did change people’s mentality. Galileo and his telescope sealed the victory for the Copernican theory, not with arguments and empirical data, but because the telescope’s role was sociological and psychological.

What is the most important aspect in Kuhn’s account? Decisive is not what he considered but what he ignored, i.e. he focused on comparing the Copernican, Ptolemaic and Tychonic systems by also taking into account various sociological and psychological factors while entirely ignoring the reliability of empirical data obtained by Galileo’s use of the telescope. The problematic reliability of the empirical data obtained by Galileo is not noticed by Kuhn, and, thus, it is not analysed at all. The telescope in Kuhn’s account plays the role of an oracle which tells us what reality truly looks like, and the observations provided by Galileo are considered to be reliable and not subject to criticism.

Difficulties with the empirical data provided by Galileo’s telescope

Galileo’s telescope provided reliable observational data only when it was used to observe objects at close range, for instance, houses, hills or landscapes. One could observe these objects enlarged and the image was high quality. But when Galileo directed the telescope to the sky, amazing things happened:

“The moon, then, seemed to be full of mountains at the inside but perfectly smooth at the periphery, and this despite the fact that the periphery changed as the result of the slight libration of the lunar body. The moon and some of the planets, such as for example Jupiter, were enlarged while the apparent diameter of the fixed stars decreased: the former were brought nearer whereas the latter were pushed away”[13].


We can easily note that Galileo’s telescope was not a completely reliable instrument – in some cases it provided clearly false data. To become convinced of this was enough to look at the sky with an unaided eye. The clearest example is the circular quirk, which had been observed by Galileo and documented in one of his drawings:

“Thus the circular monster below the centre of the disk of the moon is well above the threshold of naked eye observation (its diameter is larger than 3½ minutes of arc), while a single glance convinces us that the face of the moon is not anywhere disfigured by a blemish of this kind. It would be interesting to see what contemporary observers had to say on the matter or, if they were artists, what they had to draw on the matter”[14].

Another problem with the recognition data obtained using the telescope was that it was unable to adapt to the individual human. This means that the lens were permanently attached to the leaden pipe and different people could not adjust them to the state of their sight, for instance, Galileo could clearly see many objects in the sky, but a person suffering from myopia or foresight could se nothing[15].

Of course prejudices, conservatism and unwillingness to change old ideas negatively affected the reception of Galileo’s thesis. This should not be a surprise, as resistance to change for human beings, after all, is not unusual. And undoubtedly Galileo promoted “subversive” ideas, contrary to Aristotelian cosmology and the Ptolemaic hypothesis, which was useful in predicting the location of celestial objects. But if we ignore the impact of different types of prejudice and evaluate the reliability of empirical data obtained by using the telescope, we have to admit that Galileo did not provide any conclusive evidence for the Copernican hypothesis.

“The telescope, on the other hand, produces new and strange phenomena, some of them exposable as illusory by observation with the naked eye, some contradictory, some having even the appearance of being illusory, while the only theory that could have brought order into this chaos, Kepler’s theory of vision, is refuted by evidence of the plainest kind possible”[16].

Galileo did not provide any reliable arguments to his opponents which could explain all of the problems that appeared along with the use of a telescope to observe the sky. In 1610 the need for optical theory explaining any uncertainty associated with the image of the telescope was obvious. Only if such an optical theory had been accepted we can we start to think about the significance of new empirical data for resolving the dispute between the systems of Tycho Brahe, Ptolemy and Copernicus.

“Only a new theory of vision, containing both hypotheses concerning the behaviour of light within the telescope and hypotheses concerning the reaction of the eye under exceptional circumstances, could have bridged the gulf between the heavens and the earth that was, and still is, such an obvious fact of physics and of astronomical observation”[17].

A Crisis of the Ptolemaic system?

We have to admit that there was no crisis of the Ptolemaic system at the beginning of the seventeenth century. The geocentric theory of Ptolemy was truly adequate at predicting the position of the planets in the sky. It is possible to calculate the planetary positions with any accuracy by multiplying the epicycles[18]. This does not mean that Ptolemy’s theory was free from defects – it was far from perfect in terms of aesthetics and pragmatics. This means that scientists had to take into account many different things, such as epicycles, deferents and equants, which made any calculations of the planetary positions very complicated. And we have to remember that there were no computers in the seventeenth century to help astronomers in their work. Ptolemy’s system was also awkward due to the multiplicity of things which scientists had to take into account and which could give them the impression that the universe actually must be more simple. But this was only an impression, not a rational argument. Besides, the Copernican system assumed that planetary orbits are circular. This is untrue. Moreover, it causes that all calculations of celestial objects were inaccurate, and this raised the need for a greater number of epicycles and deferents. Hence, the Copernican system did not bring simplicity of the calculations. Simplicity and accuracy of calculations were possible when Kepler proposed that the orbits of planets should be considered as an ellipse. This was in 1609.

“Most of Galileo’s opponents behaved (…) rationally. Like Bellarmine, they agreed that phenomena were in the sky, but denied that they proved Galileo’s contention. In this, of course, they were quite right. Thought the telescope argued much, it proved nothing”[19].

Galileo’s arguments


Galileo’s behaviour in the fight against Ptolemy’s theory and promotion of Copernicus’ theory is highly controversial. He formulated two types of arguments:

1)      substantive, which shed new light on some problems. For instance, Galileo changed the definition of “motion”: formerly people thought that every motion should be easily observable, but thanks to Galileo we know that this is not true. There could be motion that is virtual. We can spot this kind of motion when we are on a vessel that is leaving a port. We then think that this port is moving away from us, not that we are moving away from the port.

2)      persuasive, which brought noting essential to the debate. For instance, Galileo said that the greatest strength of the Copernican theory was that all of its supporters had once been its enemies[20]. One may answer that most supporters of the old theory did not change their opinions after hearing the new theory. Galileo’s argument was, in fact, not an argument at all.


Feyerabend versus Kuhn

Now it is time to compare Kuhn’s and Feyerabend’s accounts. Let us consider how the Copernican Revolution is described by Thomas Kuhn:

1)      The Ptolemaic model was awkward and the Copernican system brought “aesthetical simplicity” and, after Kepler proposed that orbits of planets are elliptic, simplicity and accuracy of calculations.

2)      The empirical data obtained by using the telescope was reliable but it did not falsify old theories, especially the theory of Tycho Brahe, which was able to explain all of the new facts discovered by observing the sky through a telescope.

3)      Some opponents of the Copernican system were biased; they were afraid of destroying the old “cosmology that for centuries had been the basis of everyday practical and spiritual life”[21].

Now it is time to consider Feyerabend’s account. At the beginning of the article we distinguished five features of methodological anarchism; so how do Galileo’s activities fit into these?

1)      Galileo was working hard to find some new facts to falsify the old and well-confirmed Ptolemaic theory; that is why he directed the telescope at the sky.

2)      He promoted a theory which implies a vision of the universe which is different from the vision proposed by a well-confirmed theory. When he had no rational arguments, he used persuasive arguments and propaganda.

3)      The Copernican theory produced new facts, such as the phases of Venus. It is plausible that he supposed that sustaining and developing the Copernican theory would generate more new facts.

4)      He sustained the Copernican theory despite the lack of evidence for its confirmation; evidence obtained by the telescope was unreliable. For a theoretical monist it is obvious that only one theory could be developed and he would dismiss the new theory due to the absence of evidence confirming it. Galileo acted to the contrary.

5)      Galileo neglected reasonable methodological standards which required the abandonment of uncertain data obtained by using the telescope.

It is obvious that Galileo’s activities fit well with Feyerabend’s view of methodological anarchism.

Notice that features 1) and 3) of the Copernican Revolution, as distinguished by Kuhn, are acceptable by methodological anarchist. Feature 2) is unacceptable – Feyerabend doubts the reliability of data obtained by Galileo. But for a complex comparison we have to distinguish three different dimensions:

1)      “scope” – how many different factors they take into account. Feyerabend’s account is much richer. He critically analyses what Galileo had observed, what he knew, what the state of development of optics that could possibly explain all the problems that occurred when Galileo directed the telescope at the sky was. This is what Kuhn totally ignores – for him the telescope is a reliable source of empirical data and that statement is not justified

2)      “adequacy” – Kuhn’s account cannot be in accordance with the facts because it ignores the most overwhelming issue. And if he does not take into consideration a very important issue then he cannot accurately interpret events taking place in the early seventeenth century. Feyerabend accurately assigns Galileo’s activities as marks of a counter-inductive, anarchistic methodology

3)      “possibility of genuine judgment” – only an adequate interpretation of seventeenth century events could be a basis for assessing the behaviour of all parties to the debate, so this is another point for Feyerabend.


This article presented the views of Paul Feyerabend and Thomas Kuhn regarding the Copernican Revolution. We have argued that the Feyerabend account is more accurate than Kuhn’s relations and that an accurate account could be a basis for an evaluation of Galileo’s and the Catholic Church’s activities. Now is a good time to make such a judgment.

Feyerabend shows us that rejecting historical relativism and evaluating on the basis of our actual state of knowledge is unacceptable. Taking into account the historical context changes our common view of the events that took place in the early seventeenth century. The simple juxtaposition of “progressive Galileo/backward and intolerant Catholic Church” is no longer valid. Galileo did not present any reliable evidence which could falsify the Tychonic or Ptolemaic theory. To recognise the reliability of empirical data obtained by using a telescope, it was necessary to present the optical theory explaining all the difficulties that occurred. Galileo did not know such a theory, but he was quite effective at propaganda. Moreover, there are arguments to see the Catholic Church as open to the arguments: Roberto Bellarmino, who supervised Galileo’s trial, wrote that he was willing to accept the Copernican theory if the relevant evidence were presented. He thought that incompatibility between the Bible and science should result in a change in our interpretation of the Bible – if the Bible states that the Sun circulates the Earth and science denies it, then that means that we have to start interpreting some parts of the Bible in a symbolic and metaphorical way[22]. The Catholic Church was the defender of reasonable methodology, and if we accept that science develops rationally then we cannot blame it for condemning Galileo. But we can blame it for recognising itself as the defender of reason and being involved in nonreligious activity. If we accept methodological anarchism, we cannot blame anybody due to historical relativism.


Feyerabend, Paul Karl, 1993, Against Method: Outline of an Anarchistic Theory of Knowledge, London & New York: Verso

Galileo, 2005, Fragmenty kopernikańskie, Wydawnictwo Uniwersytetu Warszawskiego: Warszawa

Kuhn, Thomas Samuel, 1995, Copernican Revolution. Planetary Astronomy in the Development of Western Thought, Cambridge, Massachusetts&London, England: HarvardUniversity Press

Kuhn, Thomas Samuel, 1962, The Structure of Scientific Revolutions, Chicago: The University of Chicago Press

Wróblewski, Andrzej Kajetan, 2007, Historia fizyki: od czasów najdawniejszych do współczesności, Warszawa: Wydawnictwo Naukowe PWN



Bartosz Kaluziński, mgr – doktorant w Instytucie Filozofii Uniwersytetu im. Adama Mickiewicza, interesuje się filozofią analityczną (późny Wittgenstein, spór o realizm semantyczny) oraz filozofią nauki (Kuhn, Feyerabend, Lakatos)

[1] We commonly use the terms “heliocentric” and “geocentric”, but these are not precise. For example, the term “heliocentric” suggests that the centre of the solar system is in the middle of the sun, but in Ptolemy’s model this is false because the means of the circular orbits of planets do not coincide with the centre of the sun. Hence, better terms would be “heliostatic” and “geostatic”, respectively. Both of the main characters of our story, i.e. Kuhn and Feyerabend, use the commonly accepted terms, and we shall do the same in order to avoid any unnecessary complications in the terminology. See: Wróblewski, 2007, 78-85.

[2] Feyerabend, 1993, p. 20-23.

[3] Ibid., p. 14-15.

[4] Ibid., p. 24.

[5] Ibid., p. 26.

[6] Ibid., p. 51.

[7] Ibid., p. 27.

[8] Ibid., p. 17-18.

[9] Ibid., p. 15.

[10] Ibid., p. 18.

[11] Kuhn T. S., 1995, p. 220.

[12] Ibid., p. 224.

[13] Feyerabend, 1993, p. 91-92.

[14] Ibid., p. 98-99.

[15] Wróblewski, 2007, p. 102.

[16] Feyerabend, 1993, p. 103.

[17] Ibid., p. 87.

[18] Wróblewski, 2007, p. 80.

[19] Kuhn, 1995, p. 226.

[20] Galileo, 2005, p. 25.

[21] Kuhn, 1995, p. 226.

[22] Galileo, 2005, p. 127-129.