 Apart from the questions of whether operational definitions are sufficient or necessary, it is actually unclear what types of things operations are, and how they should be specified. This problem was noted and discussed quite early on (see, for example, Hearnshaw 1941). The surface-level intuition is simple: the operations that matter are measurement operations involving physical instruments. But from the start Bridgman (1927, 5) also stated that the operations which fixed meaning were mental if the concepts in question were mental (e.g., in mathematics). And he knew that measurement operations involved more than physical manipulations of instruments; at least there are recordings and computations involved in the processing and analysis of data, and there are mental acts linking various parts of that complex procedure, too. To take the simplest example, the operation of counting is a mental operation, but it is an integral part of many “physical” procedures. He called such crucial non-physical operations “paper-and-pencil” operations. Bridgman lamented that it was the “most wide spread misconception with regard to the operational technique” to think that it demanded that all concepts in physics must find their meaning only in terms of physical operations in the laboratory (Bridgman 1938, 122–124; also Bridgman 1959b, 522). Later he gave a rough classification of operations into the instrumental, mental/verbal, and paper-and-pencil varieties (Bridgman 1959a, 3).