An early theological application of Galileo’s doctrine of motion: Insights from Benedetto Castelli’s first letter to Galileo (April 1, 1607)

In 1607, Benedetto Castelli sent a letter to Galileo Galilei from Cava de’ Tirreni. This correspondence provides valuable insights into Castelli’s mathematical training and is significant for two main reasons. First, it demonstrates that by 1607, Galileo had already articulated key principles of his scientific work, such as the relativity of motion and a concept resembling inertia. Thus, the letter serves as an important source on Galileo’s advanced thinking prior to the publication of Sidereus Nuncius (1610). Second, it explores the relationship between Galileo’s ideas on motion, the eternity of the world, and the existence of God. Castelli refutes Aristotelian errors and underscores the limitations of Galilean science regarding theological matters, aiming to prevent conflicts between scientific inquiry and religious truths. This letter thus highlights the early debates sparked by Galileo’s new “doctrine” of motion, occurring before any of his major findings were published.