A watershed in the history of science, this book was a difficult one to get through, but I’m glad I read it. One of my favorite passages is as follows:
Now two other periods occur, the monthly and the annual. These do not introduce new and different events beyond those already considered under the diurnal period, but they act upon the latter by making them greater or less at different parts of the lunar month and at different seasons of the solar year—almost as though the moon and sun were taking part in the production of such effects. But that concept is completely repugnant to my mind; for seeing how this movement of the oceans is a local and sensible one, made in an immense bulk of water, I cannot bring myself to give credence to such causes as lights, warm temperatures, predominances of occult qualities, and similar idle imaginings. These are so far from being actual or possible causes of the tides that the very contrary is true. The tides are the cause of them; that is, make them occur to mentalities better equipped for loquacity and ostentation than for reflection upon and investigations into the most hidden works of nature. Rather than be reduced to offering those wise, clever, and modest words, “I do not know,” they hasten to wag their tongues and even their pens in the wildest absurdities.
After talking for 400 pages about the true motions of the earth, sun and planets, and getting everything right, Galileo begins talking about the tides and gets everything wrong. But still in his confidence he manages to ridicule the other theories that are equally wrong about the origin of the tides. They’re not caused by the slowing down and speeding up of the earth as the sun and moon tug on it as it makes its way through the heavens, they are caused by the sun and moon themselves, and the inherent force of gravity that all mass in the universe possesses, a force that was dimly understood in Galileo’s time. His comparisons of the planets in their orbits to weights swinging at the ends of pendulums is quaint, but understandable given his observational approach to solving the riddles of the universe. And his solutions for the motions of the heavenly bodies are right on target, derived as they are from observation and analytical geometry. But what makes them move? Ultimately, that answer eludes this remarkable man in his remarkable age, and calls forth another passage from earlier in the book:
And finally I ask you, O foolish man: Does your imagination first comprehend some magnitude for the universe, which you then judge to be too vast? If it does, do you like imagining that your comprehension extends beyond the Divine power? Would you like to imagine to yourself things greater than God can accomplish? And if it does not comprehend this, then why do you pass judgment upon things you do not understand?
Salviati is talking to Simplicio here, but clearly Galileo is also talking to a larger audience. He’s slamming people who claim things are impossible because they’ve decided that they are, or because they are beyond their understanding. But in a sense, that is what Galileo is doing with his theory on the tides and his contempt for the theories put forward by others. It may very well be that Galileo failed to remember his own homily:
Let us just say that there are two sorts of poetical minds—one kind apt at inventing fables, and the other disposed to believe them.