Truth and Method: The Scientific Content of Galileo’s Dialogue

Abstract

Scientists from Newton to Einstein have found Galileo’s Dialogue on the Two Chief World Systems to be rich in scientific content. It is well known, for example, that in the Principia newton attributed to Galileo a knowledge of the law of inertia, the law of force, the principle of superposition, the law of squares, and the parabolic path of projectiles;¹ what is not well known is that, as I. B. Cohen has argued, “Newton almost certainly did not read the Discorsi - if, indeed, he ever did at all - until some considerable time after he had published the Principia”,² whereas “the evidence is certain and unmistakable that Newton, early in his scientific career, had read the great Dialogo... Sopra i due massimi sistemi”.³ it is also well known that, in his foreword to drake’s translation of the Dialogue, Einstein summarizes its scientific content as being a result about the nonexistence of an abstract center of the universe, with analogies to his own work. According to Einstein, “Galileo opposes the introduction of this ‘nothing’ (center of the universe) that is yet supposed to act on material bodies; he considers this quite unsatisfactory... [because] although it accounts for the spherical shape of the earth it does not explain the spherical shape of the other heavenly bodies”.⁴ Einstein then suggests

that a close analogy exists between Galileo’s rejection of the hypothesis of a center of the universe for the explanation of the fall of heavy bodies, and the rejection of the hypothesis of an inertial system for the explanation of the inertial behavior of matter. (The latter is the basis of the theory of general relativity.) Common to both hypotheses is the introduction of a conceptual object with the following properties:

• (1). It is not assumed to be real, like ponderable matter (or a “field”).

• (2). It determines the behavior of real objects, but it is in no way affected by them.

The introduction of such conceptual elements, though not exactly inadmissable from a purely logical point of view, is repugnant to the scientific instinct.⁵