Philosophy during the Weimar Republic pursued paths of enquiry into scientific fields such as mathematics and physics. Leading scientists became associated as a group that was called the Berlin Circle. Among many influential thinkers, Carl Hempel was a strong influence in the group. Born in Berlin, Hempel attended the University of Göttingen and the University of Heidelberg, then returned to Berlin, where he was taught by influential physicists Hans Reichenbach and Max Planck, and logistics with mathematician John von Neumann. Reichenbach introduced Hempel to the Vienna Circle, who were an existing informal association of "scientifically interested philosophers and philosophically interested scientists", as Hempel put it. Hempel was intrigued by the logical positivism ideas discussed by the Vienna Circle, and he developed a similar network, the Berlin Circle. Hempel's reputation has grown to the extent that he is now considered one of the leading scientific philosophers of the 20th century. Richard von Mises was active in both groups.