various ways (e.g. it's impossible to use a measuring rod to measure the distance to the Moon), so "length" logically isn't one concept but many, some concepts requiring knowledge of geometry. Each concept is to be defined by the measuring operation used. So the criticism is that there are potentially infinite concepts, each defined by the methods that measured it, such as angle of sighting, day of the solar year, angular subtense of the moon, etc. which were gathered together, some astronomical observations taken over a period of thousands of years. In the 1930s, Harvard experimental psychologist Edwin Boring and students Stanley Smith Stevens and Douglas McGregor, struggling with the methodological and epistemological problems of defining measurement of psychological phenomena, found a solution in reformulating psychological concepts operationally, as it had been proposed in the field of physics by Bridgman, their Harvard colleague. This resulted in a series of articles that were published by Stevens and McGregor from 1935, that were widely discussed in the field of psychology and led to the Symposium on operationism in 1945, to which also Bridgman contributed. Operationalization The practical 'operational definition' is generally understood as relating to the theoretical definitions that describe reality through the use of theory. The importance of careful operationalization can perhaps be more clearly seen in the development of General Relativity. Einstein discovered that there were two operational definitions of "mass" being used by scientists: inertial, defined by applying a force and observing the acceleration, from Newton's Second Law of Motion; and gravitational, defined by putting the object on a scale or balance. Previously, no one had paid any attention to the different operations used because they always produced the same results, but the key insight of Einstein was to posit the Principle of Equivalence that the two operations would always produce the same result because they were equivalent at a deep level, and work out the implications of that assumption, which is the General Theory of Relativity. Thus, a breakthrough in science was achieved by disregarding different operational definitions of scientific measurements and realizing that they both described a single theoretical concept. Einstein's disagreement with the operationalist approach was criticized by Bridgman as follows: "Einstein did not carry over into his general relativity theory the lessons and insights he himself has taught us in his special theory." (p. 335). In the social sciences Operationalization is often used in the