There's a 1969 Lipman Bers 'Calculus' that has on p. 301: "... the kind that made Euler say: 'My pen is more intelligent than I.'"
Perhaps this is in reference to a passage in Euler's (translated) reflections on the system of monads (Letters of Euler on Different Subjects in Physics and Philosophy, 1802): "Is it not to be apprehended that the monads which compose the pen wherewith I am writing, may have ideas of the universe much clearer than those of my soul? How can I be assured of the contrary? I ought to be ashamed to employ a pen in conveying my feeble conceptions, while the monads of which it consists possibly conceive much more sublimely; and you might have greater reason to be satisfied, should the pen commit its own thoughts to paper, instead of mine."