In Structure and Interpretation of Classical Mechanics, a physics textbook by Gerald Sussman and Jack Wisdom, some amusing quotes from the chapter headers:

• “In almost all textbooks, even the best, this principle is presented so that it is impossible to understand. I have not chosen to break with tradition." — V. I. Arnold, Mathematical Methods of Classical Mechanics
• "I adhered scrupulously to the precept of that brilliant theoretical physicist L. Boltzmann, according to whom matters of elegance ought be left to the tailor and to the cobbler." — Albert Einstein, Relativity, the Special and General Theory
• "The purpose of mechanics is to describe how bodies change their position in space with “time.” I should load my conscience with grave sins against the sacred spirit of lucidity were I to formulate the aims of mechanics in this way, without serious reflection and detailed explanations. Let us proceed to disclose these sins." — Albert Einstein, Relativity, the Special and General Theory
• "The polhode rolls without slipping on the herpolhode lying in the invariable plane." — Herbert Goldstein, Classical Mechanics
• "We have done considerable mountain climbing. Now we are in the rarefied atmosphere of theories of excessive beauty and we are nearing a high plateau on which geometry, optics, mechanics, and wave mechanics meet on common ground. Only concentrated thinking, and a considerable amount of re–creation, will reveal the beauty of our subject in which the last word has not been spoken." — Cornelius Lanczos, The Variational Principles of Mechanics
• "What, then, is time? I know well enough what it is, provided that nobody asks me; but if I am asked what it is and try to explain, I am baffled. All the same I can confidently say that I know that if nothing passed, there would be no past time; if nothing were going to happen, there would be no future time; and if nothing were there would be no present time." — Augustine of Hippo, from Confessions, Book XI, Section 14, translation by R.S. Pine-Coffin
• "Having treated the motion of the moon about the earth, and having obtained an elliptical orbit, [Newton] considered the effect of the sun on the moon’s orbit by taking into account the variations of the latter. However, the calculations caused him great difficulties … Indeed, the problems he encountered were such that [Newton] was prompted to remark to the astronomer John Machin that “… his head never ached but with his studies on the moon.”" — June Barrow-Green, Poincaré and the Three Body Problem
• "An adequate notation should be understood by at least two people, one of whom may be the author." — Abdus Salam