Ehrenfest is discouraged in view of the rapid development of mathematical physics. Einstein consoles him by saying that there are different ways of pursuing physics.

Some are sticklers for principle, others are virtuosos. All three of us [Bohr, Einstein, Ehrenfest] belong to the first kind. [. . . ], so what we feel when we encounter pronounced virtuosos (Born or Debye) is discouragement. By the way, the reverse effect is similar.