The three comets that appeared between 1618 and 1619 gave rise to another heated argument, this time between Galileo and the Jesuit priest Orazio Grassi. By now, however, Galileo had been silenced by a pronouncement of the Holy Office, which had suspended Copernicus’s De revolutionibus, admitting the theory of the Earth’s mobility only as a mathematical hypothesis, not to be proclaimed as a natural truth. Consequently, Galileo could make no effective response to Grassi, who had adopted Tycho Brahe’s geo-heliocentric theory, after Galileo’s astronomical discoveries had demonstrated the erroneous nature of the Ptolemaic system. Il saggiatore [The Assayer], a work more significant than the dispute over comets, whose nature Galileo had not fully understood, is a milestone in methodology. Moreover, it proposes a new scale of values for natural philosophy, where authority and tradition, literature and books, are deemed infinitely less important that studying the world and the mathematical laws that govern it. For some bold statements on the primary and secondary qualities of bodies, Il saggiatore too was subjected to investigation by the Inquisition, which however came to nothing.
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