A mathematical problem should be difficult in order to entice us, yet not completely inaccessible, lest it mock at our efforts. It should be to us a guide post on the mazy paths to hidden truths, and ultimately a reminder of our pleasure in the successful solution.
David HilbertOne can measure the importance of a scientific work by the number of earlier publications rendered superfluous by it
David HilbertOne must be able to say at all times--instead of points, straight lines, and planes--tables, chairs, and beer mugs
David Hilbert