Every scientific theory implies that under certain conditions, certain things will happen. Every test consists in an attempt to realize these conditions, and to find out whether we can obtain a counter-example even if these conditions are realized; for example, by varying other conditions which are not mentioned in the theory. (This shows, by the way, that a ceteris paribus clause, like `all things being equal’, must not be added to a theory since it would destroy its testability.) (p. 288)
Popper, K. R. (1992). Realism and the aim of science. New York: Routledge.