The scientist Galileo Galilei (1564-1642) wrote his “Letter to the Grand Duchess Christina” in 1615. It explains the relationship between two understandings of the universe, the scientific and religious, and argues that they are compatible. Galileo contends that science deals with the world as we observe it, while religion deals with sacred texts. The letter endures today as an example of how to unite worldviews that seem incompatible. It is also an important document in the history of science. Galileo’s observations of the Sun, Moon, and planets were foundational for the development of modern astronomy and led, after his death, to widespread acceptance of the view that the Earth orbits the Sun.

Galileo’s letter focuses on two competing models of the planetary system. The Ptolemaic model, named after the Alexandrian mathematician Ptolemy (born around 100 CE), puts the Earth at the center with other heavenly bodies orbiting around it. The Copernican model, named after the Polish astronomer Nicolaus Copernicus (1473-1543), places the Sun at the center of the planetary system. (It is called the heliocentric model, from the ancient Greek word for the Sun, helios.) At the time of Galileo’s writing, the Catholic Church accepted the Ptolemaic model. Galileo’s astronomical observations convinced him that the Copernican model was correct. It was dangerous to promote views that challenged the authority of the church. In his letter, Galileo tries to defend his theory while also defending himself against the church. He didn’t succeed in the latter. The Inquisition tried and convicted him of heresy in 1633, and he spent the rest of his life under house arrest. This study guide cites the letter by paragraph number, following the edition available through Inters.org.

Galileo begins by condemning his critics claiming they care more about appearing correct than seeking the truth (Paragraphs 1-5). He takes issue with the way his critics use the Bible to support their claims. They use biblical arguments in areas where they do not apply: physical evidence and facts. They also misinterpret biblical passages, primarily by taking them literally when they contain layers of metaphor. Galileo returns to these ideas throughout the letter.

Galileo also clearly states his position in the opening section. He asserts that the Sun is at the center of the planetary system and that all the planets, including the Earth, orbit around it. Although Galileo mentions some of his other discoveries, he focuses on the organization of the solar system, because this is the argument critics have taken issue with.

The next group of paragraphs argue that it is irresponsible to interpret the Bible literally (Paragraphs 6-16). The first reason (discussed in Paragraphs 6-8) is that the Bible uses metaphor and allusion. Galileo says that a literal reading might cause people to take away meanings that are incorrect and even heretical. He writes that on a literal reading “it would be necessary to assign to God feet, hands, and eyes, as well as corporeal and human affections, such as anger, repentance, hatred, and sometimes even the forgetting of things past and ignorance of those to come” (Paragraph 7). Galileo argues that these sorts of descriptions make the Bible accessible to all people, even the uneducated. But they can’t be taken literally.

Galileo then explains that the authority on natural phenomena should be observation and experimentation, not the Bible (Paragraphs 9-16). He takes pains to clarify that the Bible is the supreme authority. However, because the Bible expresses ideas through metaphor, we can use observation to clarify what the Bible means. The Bible avoids instructing its readers about the nature of physical phenomena. Galileo cites multiple scholars to support his point, most notably St. Augustine. Augustine says that it is not within the scope of his expertise to answer questions about the shape of the universe or how it moves: taking up questions about the physical nature of the universe “is consistent neither with my leisure nor with the duty of those whom I desire to instruct in essential matters more directly conducing to their salvation and to the benefit of the holy Church” (Paragraph 12). Scriptural questions and scientific questions are separate. Galileo cites Cardinal Baronius: “The intention of the Holy Ghost is to teach us how one goes to heaven, not how heaven goes” (Paragraph 13).

The following section (Paragraphs 16-23) returns to Galileo’s detractors. He focuses on the differences between metaphorical and physical descriptions of the universe and between observation and debate. Galileo’s detractors cite the Bible to disprove his findings, but he believes citing the Bible as if it is literal is irresponsible. He argues that theology is “queen” (i.e., the most supreme area of study) because of what it’s about (the word of God), not because it has authority over every other subject (Paragraph 19). Theologians are supposed to debate what scripture means. When they turn to physical phenomena, they are bound to come to false conclusions that simply confirm whatever they want to believe.

The next section (Paragraphs 24-32) discusses the Bible. Galileo’s detractors say that because the Bible says the same things consistently, these things must be literally true. Galileo attacks this proposition from multiple angles. He first argues that, in cases where scientific evidence suggests something different from what the Bible says, it is our responsibility to “find out the true senses of holy Scripture in those passages which superficially might seem to declare differently” (Paragraph 25). He then argues that the Bible states its ideas in a way that every person can accept, which means constructing metaphors that are easy to grasp even if they aren’t literally true.

The following section (Paragraphs 33-37) concerns people who don’t understand Galileo’s argument and come to fallacious conclusions. Galileo disparages people who twist biblical passages to fit their arguments, especially when those arguments are incorrect and based on misunderstandings. He believes people must let the evidence dictate what they think instead of forcing the Bible to uphold whatever opinions they happen to have. Often, Galileo says, this is a matter of ego: People don’t want to admit they’re wrong. Galileo also clarifies a theme central to the “Letter”: We should not use biblical evidence to understand the universe in the same way we use physical evidence. Physical evidence leads to truths that we can prove, and because the Bible is necessarily true (in Galileo’s worldview), the Bible can’t contradict it. If we don’t see scientific truth in the Bible, it is because we have an insufficient understanding of scripture.

Galileo devotes the final section of the letter (Paragraph 38 to the end) to analyzing a passage from the Book of Joshua. Joshua asks God to make the Sun stand still to give the Jewish people more time to win a battle. Galileo means to show that Joshua’s miracle is consistent with a heliocentric view of the solar system. Galileo explains that the Ptolemaic system makes Joshua’s miracle impossible. If we use the Copernican system, however, “we have the literal, open, and easy sense of another statement that we read in this same miracle, that the sun stood still in the midst of the heavens; that is, in the center, where it resides” (Paragraph 43). Galileo concludes by saying that many theologians will interpret the Bible according to whatever opinion they happen to hold.