However, he rightly deemed necessary to justify himself and defend his work against shallow accusations made by people who knew virtually nothing of his science, and pretended nevertheless to refer to their interpretation of the Bible to claim that the established Aristotelian system, firm in the belief that the Earth was the center of the universe, was true despite many observations of its falsity.
The scientific information included in this Letter is very succinct, as it is a text essentially destined to be read and understood by non-scientists such as the Duchess herself. Galileo presents briefly his new conclusions about confirming Copernicus's heliocentric theory, and also diplomatically refers to his discovery of the Medicean planets, which he had named in honor of the Medici brothers who were financially supporting his work (these are the four largest satellites of Jupiter and are nowadays known as the "Galilean moons"). This mention was seemingly made to attract the sympathy of the Duchess, as well as to confirm that the Medici were in agreement with Galileo's cosmological views, thus providing him with a strong support against his critics.
Whereas Galileo does not contest that the Bible contains the ultimate truth about God, he considers that its interpretation should be viewed critically though observation of the natural world. He refers essentially to astronomy, but also makes a parallel with anatomy in that both sciences entail the study of God's creation and allow the scientists to marvel at the wonders they discover. Then, he explains that it is possible to interpret more correctly the words of the Bible in the light of the new discoveries. He justifies his faith by stating that God has given Man senses and an intellect to use in order to learn more about the work He has displayed for all to see in both the universe and within the human body itself.
Galileo's argument highlights that not all humans are intellectually equal, and that the vast majority of them is not able to understand complex matters such as the works of Nature or the texts of the Bible. In his eyes, this is only reserved to an academic elite of scholars, which he dubs "those who deserve to be separated from the herd," thus implying that most of mankind can be compared to sheep with a limited understanding of the world around them. He does not challenge the content of the Bible, let alone the existence of God, but he strongly advises that those who do not have the intellectual capacities to understand the Bible (the word of God) and the subtleties of Nature (the work of God) should steer clear of such matters whose subtleties evade them.
The Letter includes several themes that are repeated at least twice. The author's anger is clear from the start and one of those recurrent themes is that of those unjustified attacks on his work by people who do not fully understand it, but hide behind Biblical excerpts that they quote out of context, and without fully comprehending their true meaning. He accuses them of expressing groundless opinions without even trying to look any further into the science that generated the conclusions they criticize. Galileo harshly counter-attacks at the beginning of his Letter by stating bluntly that his detractors are "[s]howing a greater fondness for their own opinions than for truth they sought to deny and disprove the new things which, if they had cared to look for themselves, their