Mathematician V.I.Arnold ( according to official rumours, he missed a Fields medal because Kolomogrov did not write a letter in time) says:
Among other important things Poincare explained that 'only non-interesting problems might be formulated
unambiguously and solved completely'.
The quote is from the book "Mathematics:Frontiers and Perspectives" Edited by Arnold, Atiyah, Lax and Mazur, Published by Amer. Math. Soc. 1999.