Walter A. Shewhart Quote

The definition of random in terms of a physical operation is notoriously without effect on the mathematical operations of statistical theory because so far as these mathematical operations are concerned random is purely and simply an undefined term. The formal and abstract mathematical theory has an independent and sometimes lonely existence of its own. But when an undefined mathematical term such as random is given a definite operational meaning in physical terms, it takes on empirical and practical significance. Every mathematical theorem involving this mathematically undefined concept can then be given the following predictive form: If you do so and so, then such and such will happen.

Shewhart, Walter A.; Deming, William E. (1939). Statistical Method from the Viewpoint of Quality Control. The Graduate School, The Department of Agriculture. p. 18.