Felix Klein

From draft of a letter by Franz Brentano of 30 April 1905 to Edmund Husserl

But the most ingenious of mathematicians, such as Euler and Descartes, never considered mathematics as being an end in itself. It is not surprising, therefore, that your Felix Klein, weary of excursions into metamathematics, should turn to the technological applications of mathematics.

THE TRUE AND THE EVIDENT,
Translated from the German WAHRHEIT UND EVIDENZ (1930)
by Roderick M. Chisholm, Ilse Politzer and Kurt R. Fisher
New York : Humanities Press 1966, pp. 139-140.