QED

In dealing with mathematical problems, specialization plays, as I believe, a still more important part than generalization. Perhaps in most cases where we seek in vain the answer to a question, the cause of the failure lies in the fact that problems simpler and easier than the one in hand have been either not at all or incompletely solved. All depends, then, on finding out these easier problems, and on solving them by means of devices as perfect as possible and of concepts capable of generalization. This rule is one of the most important levers for overcoming mathematical difficulties and it seems to me that it is used almost always, though perhaps unconsciously.

David Hilbert, “Mathematical problems. Lecture delivered before the International Congress of Mathematicians at Paris in 1900”, Bulletin of the American Mathematical Society, Vol. 8, pp. 437–479 (1902)

Gevonden op Mathematics under the Microscope.