In 1616, after Galileo had already become concerned that he was a target of suspicion by the Inquisition, he received a letter from Monsignor Francesco Ingoli listing both scientific and theological arguments against Copernicanism.
For example, actual or thought experiments similar to this one had been previously discussed by Jean Buridan,[1] Nicolas Oresme,[2] Nicolaus Cusanus,[3] Clavius[4] and Giordano Bruno.
A common Aristotelian-Scholastic argument was that if the earth's surface were moving to the east, then in this experiment the projectile would land to the west of the launching point, contrary to observation.
Galileo's 1632 book Dialogue Concerning the Two Chief World Systems considered (the Second Day) all the common arguments then current against the idea that the Earth moves.
To counter such arguments the book observes that a person on a uniformly moving ship has no sense of movement and so a cannonball dropped from the top of the mast would fall directly to the foot.
To prove the point Galileo's fictional advocate Salviati proposed the experiment described below to show the classical principle of relativity according to which there is no internal observation (i.e. without, as it were, looking out the window) by which one can distinguish between a system moving uniformly from one at rest.