What was Galileo Galilei’s book The Assayer about?

Published in 1623, Il saggiatore (The Assayer) is a brilliant polemic regarding physical reality and an exposition of the scientific method.

The book originated from a controversy concerning the nature of comets that appeared in 1618, leading Galileo to engage in an exchange with Orazio Grassi, a professor of mathematics at the Collegio Romano. Within the text, Galileo famously argued that the universe is written in the language of mathematics, utilizing geometric figures such as triangles and circles. Furthermore, he established a philosophical distinction between the properties of external objects and the sensations they produce in human observers, known as the distinction between primary and secondary qualities.1