If one really understood the central point and its necessity in the construction of the world, one ought to be able to state it in one clear, simple sentence. Until we see the quantum principle with this simplicity we can well believe that we do not know the first thing about the universe, about ourselves, and about our place in the universe.
In: Francesco de Finis (ed.), Relativity, Quanta and Cosmology in the Development of the Scientific, Thought of Albert Einstein (Volume 2)