Strength of character leads us to a degenerate form of it — obstinacy. It is often very difficult in concrete case to say where the one ends and the other begins; on the other hand, it does not seem difficult to determine the difference in the abstract. Obstinacy is not a fault of the intellect; we use the term as denoting resistance to our better judgment, and that cannot be located, without involving us in a contradiction, in the intellect, which is the capacity of judgment. Obstinacy is a fault of temperament. This inflexibility of will and impatience of contradiction find their origin only in a particular kind of egotism, which sets above every other pleasure that of governing itself and others solely by its own caprice.

[Die Charakterstärke führt uns zu einer Abart derselben, dem Eigensinn. Sehr schwer ist es oft, im konkreten Falle zu sagen, wo die eine aufhört und der andere anfängt, dagegen scheint es nicht schwer, den Unterschied im Begriffe festzustellen. Eigensinn ist kein Fehler des Verstandes; wir bezeichnen damit das Widerstreben gegen bessere Einsicht, und dieses kann nicht ohne Widerspruch in den Verstand als dem Vermögen der Einsicht gesetzt werden. Der Eigensinn ist ein Fehler des Gemütes. Die Unbeugsamkeit des Willens, diese Reizbarkeit gegen fremde Einrede haben ihren Grund nur in einer besonderen Art von Selbstsucht, welche höher als alles andere das Vergnügen stellt, über sich und andere nur mit eigener Geistestätigkeit zu gebieten.]

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:

Force of character leads us to a spurious variety of it -- obstinacy. It is often very difficult in concrete cases to say where the one ends and the other begins; on the other hand, it does not seem difficult to determine the difference in idea. Obstinacy is no fault of the understanding; we use the term as denoting a resistance against our better judgment, and it would be inconsistent to charge that to the understanding, as the understanding is the power of judgment. Obstinacy is a fault of the feelings or heart. This inflexibility of will, this impatience of contradiction, have their origin only in a particular kind of egotism, which sets above every other pleasure that of governing both self and others by its own mind alone.
[tr. Graham (1873)]

Strength of character can degenerate into obstinacy.. The line between them is often hard to draw in a specific case; but surelyi it is seasy to distinguish them in theory. Obstinacy is not an intellectual defect; it comes from reluctance to admit that one is wrong. To impute this to the mind would be illogical, for the mind is the seat of judgment. Obstinacy is a fault of temperament.. Stubbornness and intolerance of contradiction result from a special kind of egotism, which elevates above everything else the pleasure of its autonomous intellect, to which others must bow.
[tr. Howard & Paret (1976)]