In general, we mean by any concept nothing more than a set of operations; the concept is synonymous with the corresponding set of operations.
Bridgman, 1927, The Logic of Modern Physics, p. 5.
(4) Exact definitions of psychological concepts are best given by functional equations. On one side of the equation stand experimental observations and operations; on the other a word or phrase designed to comprehend them.
Pratt, 1939, The Logic of Modern Psychology, p. ix.