The problem of black body radiation

In a letter to R. W. Wood Planck called his act of discretizing energy “an act of despair” and for ten years he tried to set his idea within the framework of classical physics. In that same letter he wrote:

I knew that the problem (of the equilibrium of matter and radiation) is of fundamental significance for physics; I knew the formula that reproduces the energy distribution in the normal spectrum; a theoretical interpretation had to be found at any cost, no matter how high.

(Source: Lecture notes "Introduction to theoretical physics" by Ennio Gozzi)