Galileo’s microscopic and telescopic observations and their impact on how bodies are conceived

Abstract Four hundred years ago Galileo Galilei published a work that was immensely successful directly after its publication but is relatively unknown today. The Assayer ( Il Saggiatore ) started as a polemic reply to Orazio Grassi’s interpretation of the first telescopic observation of comets in history. The Jesuit defended the idea that comets were bodies moving in the superlunary region. However, Galileo never explicitly stated that comets are an illusion or an optical defect. Rather he aimed at undermining Grassi’s argumentation. Consequently, what announced itself initially as a controversy about the nature of comets turned out to become a controversy about the nature of scientific methodology. This paper focuses on two elements that Galileo elaborates on in this pioneering work of scientific method: his views on the qualities of bodies and the fact that “the book of nature” is written in the language of mathematics. It demonstrates how these two elements are not only connected in The Assayer (1623) but also in his earlier work. In addition, it shows how they both played a role in Galileo’s defence of mathematical demonstration and his rejection of the tools of scholastic philosophy.