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“Does anyone believe that the difference between the Lebesgue and Riemann integrals can have physical significance, and that whether say, an airplane would or would not fly could depend on this difference? If such were claimed, I should not care to fly in that plane.”
― Richard W. Hamming

“The integrals which we have obtained are not only general expressions which satisfy the differential equation, they represent in the most distinct manner the natural effect which is the object of the phenomenon... when this condition is fulfilled, the integral is, properly speaking, the equation of the phenomenon; it expresses clearly the character and progress of it, in the same manner as the finite equation of a line or curved surface makes known all the properties of those forms.”
― Jean-Baptiste-Joseph Fourier

“Psychology: it's sociology for sociopaths.”
― Ben Orlin, Change Is the Only Constant: The Wisdom of Calculus in a Madcap World

“Today it is given in the textbooks as an example which young students dispose of in twenty minutes or less. Yet it held Newton up for twenty years. He finally solved it, of course”
― Eric Temple Bell, Men of Mathematics