Full Idea
The first new model of the atom could not explain the most characteristic features of the atom, its enormous stability. No planetary system following the laws of Newton's mechanics would ever go back to its original configuration after a collision.
Gist of Idea
An atom's stability after collisions needs explaining (which Newton's mechanics can't do)
Source
Werner Heisenberg (Physics and Philosophy [1958], 02)
Book Reference
Heisenberg,Werner: 'Physics and Philosophy' [Penguin 1989], p.21