The chain of cause and effect could be quantitatively verified only if the whole universe were considered as a single system — but then physics has vanished, and only a mathematical scheme remains. The partition of the world into observing and observed system prevents a sharp formulation of the law of cause and effect.

The Physical Principles of the Quantum Theory - Translated by Carl Ekhart and Frank C. Hoyt (p. 58), The University of Chicago Press. 1930