Bahnsen developed a close friendship with fellow philosopher Karl Robert Eduard von Hartmann, until they eventually fell out and became philosophical rivals. This is not surprising, as both thinkers had similar philosophical beginnings and influences, yet differed crucially in their interpretation of these influences. Hartmann placed Schopenhauer's idea of an ever-desiring will within the unconscious psyche of the subject, while accepting Hegel's underlying rationalism and historicism. Hartmann had an affinity for pantheist monism, and asserted that the will and the rational spirit were ultimately one and the same. In contrast, Bahnsen rejected Hegel's rationalism and accepted his dialectic in a negative form, while integrating it with a pluralized version of Schopenhauer's metaphysics of will. This is a rather ironic demonstration of Bahnsen's idea of "existence as contradiction", as Bahnsen and Hartmann's interest in both philosophers both drew them together "and" created contention within their friendship.