In 1961 , he became a privatdozent in Marburg , and he began working as a `` extraordinary professor '' ( professor without chair ) of philosophy at the University of Heidelberg ( at the instigation of Hans-Georg Gadamer and Karl LÃ with ) in 1962 . In 1961 , he became a privatdozent in Marburg , andâ '' in a move that was highly unusual for the German academic scene of that timeâ '' he was offered the position of `` extraordinary professor '' ( professor without chair ) of philosophy at the University of Heidelberg ( at the instigation of Hans-Georg Gadamer and Karl LÃ with ) in 1962 , which he accepted .