Galileo: How Much Did He Know and How Much Did He Presume?

In a recent post, I drew on Prof. Steven L. Goldman’s thoughts to contrast the pure inductionist Bacon with the convinced deductionist Descartes. Since Galileo was a contemporary both of Bacon and Descartes (Galileo and Bacon being almost exactly the same age, Descartes being about forty years younger), it might also be worthwhile to compare Bacon to Galileo in regard to their knowledge claims.

Writes Goldman, “What did Galileo actually know about nature? Did he, for example, know that the Earth moved on its axis and around the Sun? Did his telescope give him that knowledge? Did his immensely influential Dialogue Concerning the Two Chief World Systems prove that Copernicus was right? Did Galileo know, as he claimed he did, that four moons orbited Jupiter, that sunspots were on or near the surface of the Sun, that the Moon’s surface was uneven, that the tides were caused by the Earth’s rotation? The Catholic Church typically has been cast by historians as a villain in the condemnation of Galileo, but a great deal hinges on whether Galileo possessed knowledge and was defending truth or was promoting personal opinions based on his beliefs. Galileo employed a method of reasoning that was different both from Bacon’s and Descartes’ and was more influential than either. He was arguably the first modern mathematical physicist, deeply committed to the idea of nature as being intrinsically mathematical. He employed a rigorously objective and empirical method of reasoning that used experiment selectively to confirm the validity of idealized mathematical models of natural phenomena. He used thought experiments to reach scientific conclusions and, on occasion, extended the logical consequences of his idealized reasoning to nature as if he had actually observed experimentally what could not have been observed.”

What Galileo had in common with both Bacon and Descartes was his conviction that we can have universal, necessary and certain knowledge of nature, but he put a much greater emphasis on mathematical physics than Bacon. In that, Galileo was heavily influenced by Archimedes, some of whose works had just (in 1543) been translated in Latin and were widely studied in Italy at the time that Galileo was a student. Archimedes had applied mathematics to material phenomena, such as water pressure and the design of machines, in addition to optics and astronomy. Galileo extended this to matter in motion generally, stating that mathematics is the very language of the “book of nature” and that mathematical forms are the “alphabet” of this language.

Hence, Galileo firmly believed that “demonstration” could give us certain knowledge of nature. But “demonstration” was something completely different from Bacon’s purely inductive method. By “demonstration,” Galileo meant deductive reasoning in the manner of Euclidean geometry applied to natural phenomena. The knowledge thus derived, Galileo maintained, was identical with God’s knowledge—of course not quantitatively identical, but qualitatively. We can really partake in knowing nature as God knows nature. Somewhat anachronistically, we might say that Galileo believed that we can know das Ding an sich, “the thing in itself.”

In that, Galileo bit off more than he could chew, presuming thought experiments to correlate one hundred percent with reality, though they turned out to be not true at all, neither in a God-view way nor in the more moderate way of being a good picture of reality. Galileo presumed to know what in an experimental situation was essential and what not. So certain was he of this presumptive knowledge that he sometimes did not even bother to engage in an actual experiment. He often preferred thought experiments to actual experiments, because he recognized that the results of experiments can be misleading. He found thinking a more trustworthy tool for discovering nature than experimenting (which is not to deny that he did do several important experiments and is rightly considered a pioneer in the field of experimental science).

This caused him to sometimes uncritically extent conceptual analysis to nature or to completely embrace theories as true in a God-view way, when in fact they turned out be false in most of their details. The most famous of these theories is of course the Copernican astronomical theory, which Galileo championed without reservation. It was this conception of knowledge, rather than the particulars of Copernicus’ theory, that put Galileo at odds with some people in the Catholic Church. Galileo thought that knowledge was the same for us as it is for God. Therefore, Copernicus’ theory was not only useful for making predictions and possibly true in a provisional sort of way, but true absolutely. Any reasonable person, Galileo thought, would completely agree with Copernicus, and it was this (probably among other reasons) presumptuous epistemology that put him at odds with other people in the Church. Had Galileo claimed that Copernicus’ theory was the most effective means of making astronomical calculations, ignoring the question of physical reality, there would have been no conflict at all.

Nowadays we have enough data to conclude that the Church leaders were, for the most part, right in their provisional view on Copernicus and Galileo was wrong to assert that the Copernican system was absolutely right. Except for the (albeit important) fact that the earth moves around the sun, virtually all the details in Copernicus’ theory are wrong. And even at the time of Galileo, Copernicus’ theory was not the only possible explanation to make sense of all the facts then known.

Galileo’s Dialogues are about “two world systems,” conveniently ignoring a third “world system,” that of Tycho Brahe, which was the most widely supported astronomical theory in the early 17th century. Why did Galileo ignore this? Brahe’s theory and Copernicus’s theory are indistinguishable empirically, given the instruments available to Galileo. Galileo’s telescope was incapable of proving either that the Earth rotated on its axis or that it revolved around the Sun. The telescope revealed that Venus had phases, which may be said to prove that it orbits the Sun, but Brahe’s theory predicted that, too.

Galileo ignored Brahe because he did not believe Brahe’s theory was correct. But why? Why did he not believe it and instead fully subscribe to Copernicus, for whom the evidence was not any better than for Brahe? A possible reason was that giving credence to Brahe’s theory would have undermined the contrast between Copernicus’ theory and the clearly false Ptolemaic theory as the only options for a true theory of the heavens. Thus Galileo seems to have chosen to ignore Brahe for strategic rather than scientific reasons.

Galileo also ignored his fellow Copernican advocate Kepler’s arguments that the planets move at nonuniform speeds and in elliptical, not circular, orbits.

What all of this shows us is that assumptions are inevitable in scientific reasoning. One of Galileo’s assumptions was that circular motion was natural, which was proven to be a false assumption a decade after Galileo died. The Dutch mathematician-physicist Christian Huyghens successfully demonstrated that circular motion is forced motion, thereby altering the course of mathematical physics.

So, did Galileo discover facts, as an archaeologist discovers buried artefacts, or did he construe experience in ways that “made” new facts?

Galileo thought that nature was a book, and all he had to do was read it. Bacon thought that we were a book, and all we had to do was let nature write on us. Both believed in absolute knowledge via the route of nature. But Steven Goldman and others suggest that subsequent history has shown that both the method and the degree of scientific certainty are much more complex than that.

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