We therefore say once more that a strong mind is not one that is merely capable of strong emotions, but one that under stress of the strongest emotions keeps its balance, so that in spite for the storms within the breast, judgment and conviction can act with perfect freedom, like the needle of the compass on a storm-tossed ship.

[Wir sagen es also noch einmal: Ein starkes Gemüth ist nicht ein solches, welches bloss starker Regungen fähig ist, sondern dasjenige, welches bei den stärksten Regungen im Gleichgewicht bleibt, so dass trotz den Stürmen in der Brust der Einsicht und Ueberzeugung wie der Nadel des Kompasses auf dem sturmbewegten Schiff das feinste Spiel gestattet ist.]

Karl von Clausewitz (1780-1831) Prussian soldier, historian, military theorist

On War [Vom Kriege], Book 1, ch. 3 “On Military Genius [Der Kriegerische Genius],” (1.3) (1832) [tr. Jolles (1943)]
    (Source)

(Source (German)). Alternate translations:

We, therefore, say once more a strong mind is not one that is merely susceptible of strong excitement, but one which can maintain its serenity under the most powerful excitement; so that, in spite of the storm in the breast, the perception and judgment can act with perfect freedom, like the needle of the compass in the storm-tossed ship.
[tr. Graham (1873)]

We repeat: strength of character does not consist solely in having powerful feelings, but in maintaining one’s balance in spite of them. Even with the violence of emotion, judgment and principle must still function like a ship’s compass, which records the slightest variations however rough the sea.
[tr. Howard & Paret (1976)]