THE ASSAYER
INTRODUCTION
Galileo’s main claim to be the first of the Moderns rests on his laying the foundations of modern mechanics. Although some advances had been made by scholastic philosophers, he was the first to produce a recognisably modern, mathematical system of laws of motion, including the accelerated motion of freely falling bodies. It is clear that he had quite well worked out philosophical theories about the nature of matter, and how we can have knowledge of it. In particular, (a) he made a sharp distinction between ‘primary accidents’ of bodies, which can be described in purely mathematical terms, and ‘secondary accidents,’ such as colours, sounds, feelings, etc., which exist only in the observer; and (b) he believed that we can obtain rational knowledge of the former through a process of analysis and synthesis, calibrated against controlled experiment.
Unfortunately, Galileo didn’t put his philosophical ideas down in writing in any systematic way, and they have to be gleaned from incidental remarks. To make matters more difficult, he virtually never wrote up any of his theories as systematic treatises, but preferred other literary forms, such as the dialogue or the letter, where it is not always clear what represents his own views.
Thus his two most important scientific writings were both dialogues: the Dialogue on the Two Main World Systems of 1632 (in which he asserted that the Ptolemaic system was false, and the Copernican system true, and for which he was tried and imprisoned for heresy), and the Discourses about Two New Sciences of 1638 (in which he expounded his system of mechanics and motion).
The extracts I have selected are from an earlier work, The Assayer, of 1623. The context is that one Lothario Sarsi had written a book attacking Galileo, with the title Astronomical and Philosophical Weighing Scales. This is Galileo’s response, and he chose the title because an assayer is a government official whose job it is to test the accuracy of the weighing scales used by bankers and tradespersons: Galileo is putting to the test the accuracy of the scales Sarsi used to judge Galileo. The work takes the form of an extended letter to Monsignor Don Virginio Cesarini, Chamberlain to the Pope.
As usual (for him), Galileo wrote in Italian. It is in Volume 6 of the vast National Edition of Galileo’s works; but unfortunately this particular volume has gone missing from the Brotherton Library. Instead I have translated from the original edition of 1623, which the Brotherton Library happens to have in the Special Collections. Page numbers in square brackets refer to that edition.
It is translated into English (not wholly accurately) in S. Drake and C.D. O’Malley, Controversy on the Comets of 1618 (Pennsylvania, 1960), and in abridged form in S. Drake, Discoveries and Opinions of Galileo (New York, Doubleday, 1957).