Descartes, René (1596-1650)
It is not enough to have a good mind. The main thing is to use it well.
Discours de la Méthode. 1637.

Einstein, Albert (1879-1955)
Imagination is more important than knowledge.
On Science.

Einstein, Albert (1879-1955)
Do not worry about your difficulties in mathematics, I assure you that mine are greater.

Galton, [Sir] Francis (1822-1911)
Whenever you can, count.
In J. R. Newman (ed.) The World of Mathematics, New York: Simon and Schuster, 1956

Tolstoy, Count Lev Nikolgevich (1828-1920)
A man is like a fraction whose numerator is what he is and whose denominator is what he thinks of himself. The larger the denominator the smaller the fraction.
In H. Eves Return to Mathematical Circles, Boston: Prindle, Weber and Schmidt, 1989.