For it is not enough to have a good mind; the main thing is to apply it well. The greatest souls are capable of the greatest vices as well as the greatest virtues; and those who proceed but very slowly can make much greater progress, if they always follow the right path, than those who hurry and stray from it.

[Car ce n’est pas assez d’avoir l’esprit bon, mais le principal est de l’appliquer bien. Les plus grandes âmes sont capables des plus grands vices aussi bien que des plus grandes vertus; et ceux qui ne marchent que fort lentement peuvent avancer beaucoup davantage, s’ils suivent toujours le droit chemin, que ne font ceux qui courent et qui s’en éloignent.]

René Descartes (1596-1650) French philosopher, mathematician
Discourse on Method [Discours de la méthode], Part 1 (1637) [tr. Cottingham, Stoothoff (1985)]
    (Source)

Sometimes quoted "the main thing is to use it well." (Source (French)). Alternate translations:

For ’tis not enough to have good faculties, but the principal is, to apply them well. The greatest Souls are as capable of the greatest Vices, as of the most eminent Vertues: And those who move but very slowly, may advance much farther, if they always follow the right way; then those who run and straggle from it.
[tr. Newcombe ed. (1649)]

For to be possessed of a vigorous mind is not enough; the prime requisite is rightly to apply it. The greatest minds, as they are capable of the highest excellences, are open likewise to the greatest aberrations; and those who travel very slowly may yet make far greater progress, provided they keep always to the straight road, than those who, while they run, forsake it.
[tr. Veitch (1901)

For to be possessed of good mental powers is not sufficient; the principal matter is to apply them well. The greatest minds are capable of the greatest vices as well as of the greatest virtues, and those who proceed very slowly may, provided they always follow the straight road, really advance much faster than those who, though they run, forsake it.
[tr. Haldane, Ross (1911)]