A strong mind is one which does not lose its balance even under the most violent excitement.

[Ein starkes Gemüt ist ein solches, welches auch bei den heftigsten Regungen nicht aus dem Gleichgewicht kommt.]

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. Graham (1873)]
    (Source)

(Source (German)). Alternate translations:

A stout heart is one which does not lose its balance even under the most violent excitement.
[tr. Jolles (1943)]

A strong character is one that will not be unbalanced by the most powerful emotions.
[tr. Howard & Paret (1976)]