Mathematical ProblemsLecture delivered before the International Congress of Mathematicians at Paris in 1900 

.. As long as a branch of science offers an abundance of problems, so long is it alive; a lack of problems
foreshadows extinction or the cessation of independent development. Just as every human undertaking pursues
certain objects, so also mathematical research requires its problems. It is by the solution of problems that
the investigator tests the temper of his steel; he finds new methods and new outlooks, and gains a wider and
freer horizon. It is difficult and often impossible to judge the value of a problem correctly in advance; for the final award depends upon the gain which science obtains from the problem.. clearness and ease of comprehension I demand for a mathematical problem if it is to be perfect; for what is clear and easily comprehended attracts, the complicated repels us. Moreover 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 Hilbert 