The rigor of mathematics is not absolute - absolute rigor is an ideal, to be, like other ideals, aspired unto, forever approached, but never quite attained, for such attainment would mean that every possibility of error or indetermination, however slight, had been eliminated from idea, from symbol, and from argumentation.

The Human Worth of Rigorous Thinking: Essays and Addresses, The Human Significance of Mathematics (pp. 48M-9), Columbia University Press. 1916