In scientific experimentation we control everything that happens. We determine when it shall occur and where. We arrange circumstances and surroundings, atmosphere and temperatures; possible ways of getting in and possible ways of getting out. We take something that has been in, or put in something that has been out, and see what happens.

The Scientific Study of Human Society, Chapter III (p. 55), The University of North Carolina Press. Chapel Hill, North Carolina, USA. 1924