VI.

Galileo and the Inquisition

A Pisan professor, in Galileo’s absence, told the Medici—the ruling family of Florence as well as Galileo’s employers—that belief in a moving Earth was contrary to the Bible. Galileo immediately wrote a pamphlet for private circulation, Letter to Castelli, sketching his views on the relation of scripture and science. In December 1614 a Dominican friar denounced “Galileists” from a Florentine pulpit, and early in 1615 the Florentine Dominican convent of San Marco sent criticisms of Galileists to the Inquisition in Rome.

Galileo enlarged his Letter to Castelli into a Letter to the Grand Duchess Cristina on the correct use of biblical passages in scientific arguments, holding that the interpretation of the Bible should be adapted to increasing knowledge and warning against the danger of treating any scientific opinion as an article of Roman Catholic faith. This remarkable work of amateur theology was not published in Italy in his lifetime and had little influence on the course of events.  

A.

The Copernican System and the Church

Early in 1616 Copernican books were subjected to censorship by the church’s Index of Forbidden Books, and Jesuit cardinal Robert Bellarmine instructed Galileo that he must no longer hold or defend the opinion that Earth moves. Following a long tradition that hypotheses in astronomy were merely instruments or calculating devices, Cardinal Bellarmine had previously advised him to treat this subject only hypothetically and for scientific purposes, without taking Copernican concepts as literally true or attempting to reconcile them with the Bible. The ruling of 1616 similarly laid down that Catholics could use Copernicanism as a calculating device but could not say that it was the true system of the universe.

Galileo remained silent on the subject for years, working on a method of determining longitudes (see Latitude and Longitude) at sea by using his predictions of the positions of Jupiter’s satellites, resuming his earlier studies of falling bodies, and setting forth his views on scientific reasoning in a book on comets, The Assayer, which is a classic of polemical writing.

In 1624 Galileo began a book he wished to call Dialogue on the Tides, in which he discussed the Ptolemaic and Copernican hypotheses in relation to the physics of tides. In 1630 the book was licensed for printing by Roman Catholic censors at Rome, but they altered the title to Dialogue on the Two Chief World Systems. It was published at Florence in 1632. Despite the book’s having two official licenses, Galileo was summoned to Rome by the Inquisition to stand trial for “grave suspicion of heresy.”

B.

Galileo’s Trial

Although Galileo had made considerable efforts to conform to the letter of the ruling of 1616, he had clearly written a pro-Copernican book. He had occasionally also slipped up by explicitly treating the Copernican system as “probable,” meaning that, although it was yet unproven, sooner or later it could well be shown to be true. Such a position was incompatible with the ruling of 1616, as was pointed out at his trial: Catholics were allowed to use Copernicanism as a helpful calculating device, provided that they did not treat it as having any truth. 

The charge against Galileo was grounded on a report that Galileo had been personally ordered in 1616 not to discuss Copernicanism either orally or in writing. Cardinal Bellarmine had died, but Galileo produced a certificate signed by the cardinal, stating that Galileo had been subjected to no further restriction than applied to any Roman Catholic under the 1616 edict. No signed document contradicting this was ever found, but Galileo was nevertheless compelled in 1633 to abjure (formally renounce his beliefs) and was sentenced to life imprisonment (swiftly commuted to permanent house arrest). The Dialogue was ordered to be burned, and the sentence against him was to be read publicly in every university.