There is no reason why an extraphysical general principle is necessarily to be avoided, since such principles could conceivably serve as useful working hypotheses. For the history of scientific research is full of examples in which it was very fruitful indeed to assume that certain objects or elements might be real, long before any procedures were known which would permit them to be observed directly.

(January 15 1952). "A Suggested Interpretation of the Quantum Theory in Terms of "Hidden" Variables. II". Physical Review 35 (2): 188.