Dialogue Concerning the Two Chief World Systems

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Galileo’s Dialogue Concerning the Two Chief World Systems, published in Florence in 1632, was the most proximate cause of his being brought to trial before the Inquisition. Using the dialogue form, a genre common in classical philosophical works, Galileo masterfully demonstrates the truth of the Copernican system over the Ptolemaic one, proving, for the first time, that the earth revolves around the sun. Its influence is incalculable. The Dialogue is not only one of the most important scientific treatises ever written, but a work of supreme clarity and accessibility, remaining as readable now as when it was first published. This edition uses the definitive text established by the University of California Press, in Stillman Drake’s translation, and includes a Foreword by Albert Einstein and a new Introduction by J. L. Heilbron.J. L. Heilbron is a professor of history and Vice Chancellor Emeritus, University of California at Berkeley, and currently Senior Research Fellow, Worcester College, Oxford. He is the author of numerous books on the history of science, including most recently The Sun in the Church: Cathedrals as Solar Observatories and Geometry Civilized: History, Culture, and Technique.

Stephen Jay Gould is the Alexander Agassiz professor of zoology and professor of geology at Harvard and the Vincent Astor visiting professor of biology at New York University. Recent books include Full House, Dinosaur in a Haystack, and Questioning the Millennium. He lives in Cambridge, Massachusetts, and New York City.The First Day

Interlocutors Salviati, Sagredo, and Simplicio

Salviati. Yesterday we resolved to meet today and discuss as clearly and in as much detail as possible the character and the efficacy of those laws of nature which up to the present have been put forth by the partisans of the Aristotelian and Ptolemaic position on the one hand, and by the followers of the Copernican system on the other. Since Copernicus places the earth among the movable heavenly bodies, making it a globe like a planet, we may well begin our discussion by examining the Peripatetic steps in arguing the impossibility of that hypothesis; what they are, and how great is their force and effect. For this it is necessary to introduce into nature two substances which differ essentially. These are the celestial and the elemental, the former being invariant and eternal; the latter, temporary and destructible. This argument Aristotle treats in his book De Caelo, introducing it with some discourses dependent upon certain general assumptions, and afterwards confirming it by experiments and specific demonstrations. Following the same method, I shall first propound, and then freely speak my opinion, submitting myself to your criticisms-particularly those of Simplicio, that stout champion and defender of Aristotelian doctrines.

The first step in the Peripatetic arguments is Aristotle’s proof of the completeness and perfection of the world. For, he tells us, it is not a mere line, nor a bare surface, but a body having length, breadth, and depth. Since there are only these three dimensions, the world, having these, has them all, and, having the Whole, is perfect. To be sure, I much wish that Aristotle had proved to me by rigorous deductions that simple length constitutes the dimension which we call a line, which by the addition of breadth becomes a surface; that by further adding altitude or depth to this there results a body, and that after these three dimensions there is no passing farther-so that by these three alone, completeness, or, so to speak, wholeness is concluded. Especially since he might have done so very plainly and speedily.

Simp. What about the elegant demonstrations in the second, third, and fourth texts, after the definition of "continuous"? Is it not there first proved that there are no more than three dimensions, since Three is everything, and everywhere? And is this not confirmed by the doctrine and authority of the Pythagoreans, who say that all things are determined by three-beginning, middle, and end-which is the number of the Whole? Also, why leave out another of his reasons; namely, that this number is used, as if by a law of nature, in sacrifices to the gods? Furthermore, is it not dictated by nature that we attribute the title of "all" to those things that are three, and not less? For two are called "both," and one does not say "all" unless there are three.US

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