It was the prevailing system of accepting scientific facts at that time which Galileo Galilei opposed. a system to which, instead of letting people tries to discover and learn the nature and processes that surround them. it was left to those with authority, who in this case was Aristotle. Criticizing Aristotle was almost the same as criticizing the church (William).
Galileo contends that God has given man knowledge and reasoning so that he will be able to find answers to questions that come to his mind by endowing man “with senses, reason, and intellect…to give us knowledge which we can attain by them” (Letter to the Most Serene Grand Duchess). And so it is the nature of man to question things that happen around him. It is not surprising that Galileo’s thirst for knowledge about the natural world resulted in his discovery of facts leading from accepted knowledge by the Church during his time. In Part 13.5 of “Documents in Western Civilization”, Galileo’s wish to know about the formation of clouds eventually led him to discover how water vapor forms. During those periods, the only truth being accepted by the Church is that clouds are formed by water vapor. He argues that Aristotelian thinking and philosophizing based on religious scripture is nothing if the man fails to discover the properties of everything around him. As he said, “And finally by elevating us to the ultimate end of our labors, which is the love of the divine Artificer, this will keep us steadfast in the hope that we shall learn every other truth in Him, the source of all light and verify” (Galileo Galilei). In chapter 13.6, his letter to the Most Serene Duchess Mother showed his frustrations against the non-acceptance of his discoveries and line of scientific inquiry, particularly his discoveries of the heavens. . .