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• I esteem myself happy to have as great an ally as you in my search for truth. I will read your work ... all the more willingly because I have for many years been a partisan of the Copernican view because it reveals to me the causes of many natural phenomena that are entirely incomprehensible in the light of the generally accepted hypothesis. To refute the latter I have collected many proofs, but I do not publish them, because I am deterred by the fate of our teacher Copernicus who, although he had won immortal fame with a few, was ridiculed and condemned by countless people (for very great is the number of the stupid).
  • Letter to Johannes Kepler (1596), as quoted in The Story of Civilization : The Age of Reason Begins, 1558-1648 (1935) by Will Durant, p. 603

• What has philosophy got to do with measuring anything? It's the mathematicians you have to trust, and they measure the skies like we measure a field.
  • "Matteo" in Concerning the New Star (1606)

• My dear Kepler, what would you say of the learned here, who, replete with the pertinacity of the asp, have steadfastly refused to cast a glance through the telescope? What shall we make of this? Shall we laugh, or shall we cry?
  • Letter to Johannes Kepler (1610), as quoted in The Crime of Galileo (1955) by Giorgio De Santillana

• sì perché l'autorità dell'opinione di mille nelle scienze non val per una scintilla di ragione di un solo, sì perché le presenti osservazioni spogliano d'autorità i decreti de' passati scrittori, i quali se vedute l'avessero, avrebbono diversamente determinato.
  • for in the sciences the authority of thousands of opinions is not worth as much as one tiny spark of reason in an individual man. Besides, the modern observations deprive all former writers of any authority, since if they had seen what we see, they would have judged as we judge.
    • Third letter on sunspots (December 1612) to Mark Wesler (1558 - 1614), as quoted in Discoveries and Opinions of Galileo (1957) by Stillman Drake, p. 134 - 135; Italian text online at Liber Liber, also from IntraText.
  • Variant translation: In questions of science the authority of a thousand is not worth the humble reasoning of a single individual.
    • As quoted in Biographies of Distinguished Scientific Men (1859) by François Arago, as translated by Baden Powell, Robert Grant, and William Fairbairn, p. 365

• Philosophy is written in that great book which ever lies before our eyes — I mean the universe — but we cannot understand it if we do not first learn the language and grasp the symbols, in which it is written. This book is written in the mathematical language, and the symbols are triangles, circles and other geometrical figures, without whose help it is impossible to comprehend a single word of it; without which one wanders in vain through a dark labyrinth.
  • The Assayer (1623), as translated by Thomas Salusbury (1661), p. 178, as quoted in The Metaphysical Foundations of Modern Science (2003) by Edwin Arthur Burtt, p. 75
  • Variant translations:
  • Philosophy is written in this grand book — I mean the universe — which stands continually open to our gaze, but it cannot be understood unless one first learns to comprehend the language in which it is written. It is written in the language of mathematics, and its characters are triangles, circles, and other geometric figures, without which it is humanly impossible to understand a single word of it; without these, one is wandering about in a dark labyrinth.
    • As translated in The Philosophy of the Sixteenth and Seventeenth Centuries (1966) by Richard Henry Popkin, p. 65

• I have been in my bed for five weeks, oppressed with weakness and other infirmities from which my age, seventy four years, permits me not to hope release. Added to this ( proh dolor! ) the sight of my right eye — that eye whose labors (dare I say it) have had such glorious results — is for ever lost. That of the left, which was and is imperfect, is rendered null by continual weeping.
  • Letter to Élie Diodati (4 July 1637), as translated in The Private Life of Galileo : Compiled primarily from his correspondence and that of his eldest daughter, Sister Maria Celeste (1870) by Mary Allan-Olney, p. 278

• Alas! Your dear friend and servant Galileo has been for the last month hopelessly blind; so that this heaven, this earth, this universe, which I by my marvelous discoveries and clear demonstrations had enlarged a hundred thousand times beyond the belief of the wise men of bygone ages, henceforward for me is shrunk into such a small space as is filled by my own bodily sensations.
  • Letter to Élie Diodati (2 January 1638), as translated in The Private Life of Galileo : Compiled primarily from his correspondence and that of his eldest daughter, Sister Maria Celeste (1870) by Mary Allan-Olney, p. 279

• See now the power of truth; the same experiment which at first glance seemed to show one thing, when more carefully examined, assures us of the contrary.
  • Discourses and Mathematical Demonstrations Relating to Two New Sciences (1638); Discorsi e dimostrazioni matematiche, intorno à due nuove scienze, as translated by Henry Crew and Alfonso de Salvio (1914)

• You cannot teach a man anything, you can only help him to find it within himself.
  • As quoted in How to Win Friends and Influence People (1935) by Dale Carnegie, p. 117; also paraphrased as "You cannot teach a man anything, you can only help him to find it for himself."

• I have never met a man so ignorant that I could not learn something from him.
  • As quoted in The Story of Civilization : The Age of Reason Begins, 1558-1648 (1935) by Will Durant, p. 605
  • Variant translation: I never met a man so unknowledgeable, I could not learn something from him.

• Light held together by moisture.
  • His description of wine, as quoted in Discoveries and Opinions of Galileo (1957) by Stillman Drake, p. 5

• Names and attributes must be accommodated to the essence of things, and not the essence to the names, since things come first and names afterwards.
  • As quoted in Discoveries and Opinions of Galileo (1957) by Stillman Drake, p. 92
  • Variant translation: Names and attributes must be accommodated to the essence of things, and not the essence to the names, because things came first, and their names subsequently.

• Mathematics is the language with which God has written the universe.
  • As quoted in Beginning Algebra (1992) by Margaret L. Lial, Charles David Miller and E. John Hornsby, p. 2

• All truths are easy to understand once they are discovered; the point is to discover them.
  • As quoted in Angels in the workplace : stories and inspirations for creating a new world of work (1999) by Melissa Giovagnoli

• Mathematics is the key and door to the sciences.
  • As quoted in Building Fluency Through Practice and Performance (2008) by Timothy Rasinski and Lorraine Griffith, p. 64

• Measure what is measurable, and make measurable what is not so.
  • As quoted in Building Fluency Through Practice and Performance (2008) by Timothy Rasinski and Lorraine Griffith, p. 64

Letter to the Grand Duchess Christina (1615)

Essay published in 1615, in response to enquiries of Christina of Tuscany, as quoted in Aspects of Western Civilization : Problems and Sources in History (1988) by Perry McAdow Rogers, p. 53

• Some years ago, as Your Serene Highness well knows, I discovered in the heavens many things that had not been seen before our own age. The novelty of these things, as well as some consequences which followed from them in contradiction to the physical notions commonly held among academic philosophers, stirred up against me no small number of professors — as if I had placed these things in the sky with my own hands in order to upset nature and overturn the sciences. They seemed to forget that the increase of known truths stimulates the investigation, establishment, and growth of the arts; not their diminution or destruction.

• The passage of time has revealed to everyone the truths that I previously set forth; and, together with the truth of the facts, there has come to light the great difference in attitude between those who simply and dispassionately refused to admit the discoveries to be true, and those who combined with their incredulity some reckless passion of their own. Men who were well grounded in astronomical and physical science were persuaded as soon as they received my first message. There were others who denied them or remained in doubt only because of their novel and unexpected character, and because they had not yet had the opportunity to see for themselves. These men have by degrees come to be satisfied. But some, besides allegiance to their original error, possess I know not what fanciful interest in remaining hostile not so much toward the things in question as toward their discoverer. No longer being able to deny them, these men now take refuge in obstinate silence, but being more than ever exasperated by that which has pacified and quieted other men, they divert their thoughts to other fancies and seek new ways to damage me. … To this end they make a shield of their hypocritical zeal for religion. They go about invoking the Bible, which they would have minister to their deceitful purposes. Contrary to the sense of the Bible and the intention of the holy Fathers, if I am not mistaken, they would extend such authorities until even in purely physical matters — where faith is not involved — they would have us altogether abandon reason and the evidence of our senses in favor of some biblical passage, though under the surface meaning of its words this passage may contain a different sense.

• Persisting in their original resolve to destroy me and everything mine by any means they can think of, these men are aware of my views in astronomy and philosophy. They know that as to the arrangement of the parts of the universe, I hold the sun to be situated motionless in the center of the revolution of the celestial orbs while the earth revolves about the sun. They know also that I support this position not only by refuting the arguments of Ptolemy and Aristotle, but by producing many counter-arguments; in particular, some which relate to physical effects whose causes can perhaps be assigned in no other way. In addition there are astronomical arguments derived from many things in my new celestial discoveries that plainly confute the Ptolemaic system while admirably agreeing with and confirming the contrary hypothesis.
  • Variant translation: I hold that the Sun is located at the centre of the revolutions of the heavenly orbs and does not change place, and that the Earth rotates on itself and moves around it. Moreover ... I confirm this view not only by refuting Ptolemy's and Aristotle's arguments, but also by producing many for the other side, especially some pertaining to physical effects whose causes perhaps cannot be determined in any other way, and other astronomical discoveries; these discoveries clearly confute the Ptolemaic system, and they agree admirably with this other position and confirm it.

• I do not feel obliged to believe that the same God who has endowed us with senses, reason, and intellect has intended us to forgo their use and by some other means to give us knowledge which we can attain by them.

• I would say here something that was heard from an ecclesiastic of the most eminent degree: "The intention of the Holy Spirit is to teach us how one goes to heaven, not how the heavens go."
  • Variant translation: I would say here something that was heard from an ecclesiastic of the most eminent degree: "That the intention of the Holy Ghost is to teach us how one goes to heaven, not how heaven goes."

• Nature … is inexorable and immutable; she never transgresses the laws imposed upon her, or cares a whit whether her abstruse reasons and methods of operation are understandable to men. For that reason it appears that nothing physical which sense–experience sets before our eyes, or which necessary demonstrations prove to us, ought to be called in question (much less condemned) upon the testimony of biblical passages which may have some different meaning beneath their words. For the Bible is not chained in every expression to conditions as strict as those which govern all physical effects; nor is God any less excellently revealed in Nature's actions than in the sacred statements of the Bible.

Letter to Francesco Ingoli (1624)

Letter to Francesco Ingoli (1578-1649), as translated in Galileo at Work : His Scientific Biography (1978) by Stillman Drake, p. 294

• Whence do you have it that the terrestrial globe is so heavy? For my part, either I do not know what heaviness is, or the terrestrial globe is neither heavy nor light, as likewise all other globes of the universe. Heaviness to me (and I believe to Nature) is that innate tendency by which a body resists being moved from its natural place and by which, when forcibly removed therefrom, it spontaneously returns there. Thus a bucketful of water raised on high and set free, returns to the sea; but who will say that the same water remains heavy in the sea, when being set free there, does not move?

• I tell you that if natural bodies have it from Nature to be moved by any movement, this can only be circular motion, nor is it possible that Nature has given to any of its integral bodies a propensity to be moved by straight motion. I have many confirmations of this proposition, but for the present one alone suffices, which is this. I suppose the parts of the universe to be in the best arrangement, so that none is out of its place, which is to say that Nature and God have perfectly arranged their structure. This being so, it is impossible for those parts to have it from Nature to be moved in straight, or in other than circular motion, because what moves straight changes place, and if it changes place naturally, then it was at first in a place preternatural to it, which goes against the supposition. Therefore, if the parts of the world are well ordered, straight motion is superfluous and not natural, and they can only have it when some body is forcibly removed from its natural place, to which it would then return by a straight line, for thus it appears that a part of the earth does [move] when separated from its whole. I said "it appears to us," because I am not against thinking that not even for such an effect does Nature make use of straight line motion.
  • A note on this statement is included by Stillman Drake in his Galileo at Work, His Scientific Biography (1981): Galileo adhered to this position in his Dialogue at least as to the "integral bodies of the universe." by which he meant stars and planets, here called "parts of the universe." But he did not attempt to explain the planetary motions on any mechanical basis, nor does this argument from "best arrangement" have any bearing on inertial motion, which to Galileo was indifference to motion and rest and not a tendency to move, either circularly or straight.

Dialogue Concerning the Two Chief World Systems (1632)

A dialogue between fictional characters Sagredo (named after Galileo's friend Giovanni Francesco Sagredo), Salviati (named after Galileo's freind Filippo Salviati) and Simplicio (named after long dead philosopher Simplicius of Cilicia), from the translations by Stilman Drake (1953), unless otherwise noted.

• I cannot without great astonishment — I might say without great insult to my intelligence — hear it attributed as a prime perfection and nobility of the natural and integral bodies of the universe that they are invariant, immutable, inalterable, etc., while on the other hand it is called a great imperfection to be alterable, generable, mutable, etc. For my part I consider the earth very noble and admirable precisely because of the diverse alterations, changes, generations, etc. that occur in it incessantly. If, not being subject to any changes, it were a vast desert of sand or a mountain of jasper, or if at the time of the flood the waters which covered it had frozen, and it had remained an enormous globe of ice where nothing was ever born or ever altered or changed, I should deem it a useless lump in the universe, devoid of activity and, in a word, superfluous and essentially non-existent. This is exactly the difference between a living animal and a dead one; and I say the same of the moon, of Jupiter, and of all other world globes.
  The deeper I go in considering the vanities of popular reasoning, the lighter and more foolish I find them. What greater stupidity can be imagined than that of calling jewels, silver, and gold "precious," and earth and soil "base"? People who do this ought to remember that if there were as great a scarcity of soil as of jewels or precious metals, there would not be a prince who would not spend a bushel of diamonds and rubies and a cartload of gold just to have enough earth to plant a jasmine in a little pot, or to sow an orange seed and watch it sprout, grow, and produce its handsome leaves, its fragrant flowers, and fine fruit. It is scarcity and plenty that make the vulgar take things to be precious or worthless; they call a diamond very beautiful because it is like pure water, and then would not exchange one for ten barrels of water. Those who so greatly exalt incorruptibility, inalterability, etc. are reduced to talking this way, I believe, by their great desire to go on living, and by the terror they have of death. They do not reflect that if men were immortal, they themselves would never have come into the world. Such men really deserve to encounter a Medusa's head which would transmute them into statues of jasper or of diamond, and thus make them more perfect than they are.
  • Sagredo
  • Variant translation: I cannot without great wonder, nay more, disbelief, hear it being attributed to natural bodies as a great honor and perfection that they are impassable, immutable, inalterable, etc.: as conversely, I hear it esteemed a great imperfection to be alterable, generable, and mutable. It is my opinion that the earth is very noble and admirable by reason of the many and different alterations, mutations, and generations which incessantly occur in it. And if, without being subject to any alteration, it had been one great heap of sand, or a mass of jade, or if, since the time of the deluge, the waters freezing which covered it, it had continued an immense globe of crystal, wherein nothing had ever grown, altered, or changed, I should have esteemed it a wretched lump of no benefit to the Universe, a mass of idleness, and in a word superfluous, exactly as if it had never been in Nature. The difference for me would be the same as between a living and a dead creature. I say the same concerning the Moon, Jupiter, and all the other globes of the Universe.
    The more I delve into the consideration of the vanity of popular discourses, the more empty and simple I find them. What greater folly can be imagined than to call gems, silver, and gold noble, and earth and dirt base? For do not these persons consider that if there were as great a scarcity of earth as there is of jewels and precious metals, there would be no king who would not gladly give a heap of diamonds and rubies and many ingots of gold to purchase only so much earth as would suffice to plant a jessamine in a little pot or to set a tangerine in it, that he might see it sprout, grow up, and bring forth such goodly leaves, fragrant flowers, and delicate fruit? It is scarcity and plenty that makes things esteemed and despised by the vulgar, who will say that there is a most beautiful diamond, for it resembles a clear water, and yet would not part from it for ten tons of water. 'These men who so extol incorruptibility, inalterability, and so on, speak thus, I believe, out of the great desire they have to live long and for fear of death, not considering that, if men had been immortal, they would not have come into the world. These people deserve to meet with a Medusa's head that would transform them into statues of diamond and jade, that so they might become more perfect than they are.
    • Part of this passage, in Italian, I detrattori della corruptibilitá meriterebber d'esser cangiati in statue., has also ben translated into English as "Detractors of corruptibility deserve being turned into statues."
      • Dialogo sopra i due massimi sistemi del mondo. (PDF), Le Opere di Galileo Galilei vol. VII, pg. 58.

• If you could see the earth illuminated when you were in a place as dark as night, it would look to you more splendid than the moon.
  • Salviati, p. 88

• In the long run my observations have convinced me that some men, reasoning preposterously, first establish some conclusion in their minds which, either because of its being their own or because of their having received it from some person who has their entire confidence, impresses them so deeply that one finds it impossible ever to get it out of their heads. Such arguments in support of their fixed idea as they hit upon themselves or hear set forth by others, no matter how simple and stupid these may be, gain their instant acceptance and applause. On the other hand whatever is brought forward against it, however ingenious and conclusive, they receive with disdain or with hot rage — if indeed it does not make them ill. Beside themselves with passion, some of them would not be backward even about scheming to suppress and silence their adversaries.
  • p. 322

• Among all the great men who have philosophized about this remarkable effect, I am more astonished at Kepler than at any other. Despite his open and acute mind, and though he has at his fingertips the motions attributed to the earth, he nevertheless lent his ear and his assent to the moon's dominion over the waters, to occult properties, and to such puerilities.
  • In regard to Kepler's belief of the moon affecting the tides of the Earth, p. 328

• Surely, God could have caused birds to fly with their bones made of solid gold, with their veins full of quicksilver, with their flesh heavier than lead, and with their wings exceedingly small. He did not, and that ought to show something. It is only in order to shield your ignorance that you put the Lord at every turn to the refuge of a miracle.
  • As quoted in The Crime of Galileo (1976) by Giorgio De Santillana, p. 167

Letter to Fr. Vincenzo Renieri (c. 1633)

Letter to Galileo's "intimate friend and disciple, the Father Vincenzo Renieri" (1606-1647), who was chair of the mathematics department at the University of Pisa from 1640 to 1647, as quoted in A Selection from Italian Prose Writers : with a double translation: for the use of students of the Italian language on the Hamiltonian system (1828)

• After the publication of my dialogues, I was summoned to Rome by the Congregation of the holy Office, where, being arrived on the 10th of February 1633, I was subjected to the infinite clemency of that tribunal, and of the Sovereign Pontiff, Urban the Eighth; who, notwithstanding, thought me deserving of his esteem.
  • pp. 145-146

• I am certainly interested in a tribunal in which, for having used my reason, I was deemed little less than a heretic. Who knows but men will reduce me from the profession of a philosopher to that of historian of the Inquisition! But they behave to me in order that I may become the ignoramus and the fool of Italy...
  • p. 244

• I was obliged to retract, like a good Catholic, this opinion of mine; and as a punishment my dialogue was prohibited; and after five months being dismissed from Rome (at the time that the city of Florence was infected with plague), the habitation which with generous pity was assigned to me, was that of the dearest friend I had in Siena, Monsignor the Archbishop Piccolomini, whose most agreeable conversation I enjoyed with such quite and satisfaction of mind, that having there resumed my studies, I discovered and demonstrated a great number on the mechanical conclusions on the resistance of solids … after about five months, the pestilence having ceased, the confinement of that house was changed by His Holiness for the freedom of the country so agreeable to me, whence I returned to the villa of Bellosguardo, and afterwards to Arcetri, where I still breathe salubrious air near my dear native-country Florence. Stay sane.
  • p. 251-253

Letter to Giovanni Battista Baliani (1639)

Letter to Giovanni Battista Baliani (1 August 1639), as translated in Galileo at Work : His Scientific Biography (1978) by Stillman Drake, p. 399 - 401

• It now remains that we find the amount of time of descent through the channel. This we shall obtain from the marvelous property of the pendulum, which is that it makes all its vibrations, large or small, in equal times. This requires, once and for all, that two or three or four patient and curious friends, having noted a fixed star that stands against some fixed marker, taking a pendulum of any length, shall go counting its vibrations during the whole time of return of the fixed star to its original point, and this will be the number of vibrations in 24 hours. From the number of these we can find the number of vibrations of any other pendulums, longer or shorter, at will, so that if for example those counted by us in 24 hours were 234,567, then taking another shorter pendulum with which one counts 800 vibrations while another counts 150 of the longer pendulum, we already have, by the golden rule, the number of vibrations for the whole time of 24 hours; and if we want to know the time of descent through the channel, we can easily find not only the minutes, seconds, and sixtieths of seconds, but beyond that as we please. It is true that we can pass a more exact measure by having observed the flow of water through a thin passage, for by collecting this and having weighed what passes in one minute, for example, then by weighing what passes in the time of descent through the channel we can find the most exact measure and quantity of this time, especially by making use of a balance so precise as to weigh one sixtieth of a grain.

• If I shall have sufficient strength to improve and amplify what was written and published by me up to now about motion by adding some little speculations, and in particular those relating to the force of percussion, in the investigation of which I have consumed hundreds and thousands of hours, and finally reduced this to very easy explanation, so that people can understand it in less than half an hour of time.

Disputed

• Eppur si muove
  • "And yet it moves" or "but it moves" is a comment he is alleged to have made in regard to the Earth after his recantation before the Inquisition. Giuseppe Baretti was apparently the first person to record the story.

Quotes about Galilei

• It has always hurt me to think that Galilei did not acknowledge the work of Kepler … That, alas, is vanity … You find it in so many scientists.
  • Albert Einstein, in an interview with I. Bernard Cohen, as quoted in Coming of Age in the Milky Way (1988) by Timothy Ferris