"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."

"It is surely harmful to souls to make it a heresy to believe what is proved."

"In the sciences, the authority of thousands of opinions is not worth as much as one tiny spark of reason in an individual man."

"Who indeed will set bounds to human ingenuity? Who will assert that everything in the universe capable of being perceived is already discovered and known?"

"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."

"By denying scientific principles, one may maintain any paradox."

"With regard to matters requiring thought: the less people know and understand about them, the more positively they attempt to argue concerning them."

"In regard to the philosophers, if they be true philosophers, i.e., lovers of truth, they should not be irritated that the earth moves. Rather, if they realize that they have held a false belief, they should thank those have shown them the truth; and if their opinion stands firm that the earth doesn't move, they will have reason to boast than be angered."

"Measure what can be measured, and make measurable what cannot be measured."

"Nature...does not act by means of many things when it can do so by means of a few."

"It is a beautiful and delightful sight to behold the body of the Moon."

"There are those who reason well, but they are greatly outnumbered by those who reason badly."

"Wine is sunlight, held together by water."

"I wish, my dear Kepler, that we could have a good laugh together at the extraordinary stupidity of the mob. What do you think of the foremost philosophers of this University? In spite of my oft-repeated efforts and invitations, they have refused, with the obstinacy of a glutted adder, to look at the planets or the Moon or my glass [telescope]."

"The number of people that can reason well is much smaller than those that can reason badly. If reasoning were like hauling rocks, then several reasoners might be better than one. But reasoning isn't like hauling rocks, it's like, it's like racing, where a single, galloping Barbary steed easily outruns a hundred wagon-pulling horses."

"The sun, with all those planets revolving around it and dependent on it, can still ripen a bunch of grapes as if it had nothing else in the universe to do."

"In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual."

"Mathematics is the key and door to the sciences."

"It seems to me that it was well said by Madama Serenissima, and insisted on by your reverence, that the Holy Scripture cannot err, and that the decrees therein contained are absolutely true and inviolable. But I should have in your place added that, though Scripture cannot err, its expounders and interpreters are liable to err in many ways; and one error in particular would be most grave and most frequent, if we always stopped short at the literal signification of the words."

"Oh, my dear Kepler, how I wish that we could have one hearty laugh together. Here, at Padua, is the principal professor of philosophy, whom I have repeatedly and urgently requested to look at the moon and planets through my glass, [telescope] which he pertinaciously refuses to do. Why are you not here? what shouts of laughter we should have at this glorious folly! and to hear the professor of philosophy at Pisa laboring before the grand duke with logical arguments, as if with magical incantations, to charm the new planets out of the sky."

"Long experience has taught me this about the status of mankind with regard to matters requiring thought: the less people know and understand about them, the more positively they attempt to argue concerning them, while on the other hand to know and understand a multitude of things renders men cautious in passing judgment upon anything new."

"I do not think it is necessary to believe that the same God who has given us our senses, reason, and intelligence wished us to abandon their use, giving us by some other means the information that we could gain through them."

"Two truths cannot contradict one another."

"Nature is relentless and unchangeable, and it is indifferent as to whether its hidden reasons and actions are understandable to man or not."