156 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



required. To find the length of an object, we have to per- 

 form certain physical operations. The concept of length is 

 therefore fixed when the operations by which length is 

 measured are fixed: that is, the concept of length involves 

 as much as and nothing more than the set of operations by 

 which length is determined. In general, we mean by any 

 concept nothing more than a set of operations; the concept 

 is synonymous with the corresponding set of operations." l 

 Bridgman illustrates further by pointing out that the word 

 "length' means something different according as it meas- 

 ures a concrete object such as a house, a moving street car, 

 a very large object (for which a theodolite is required), the 

 distance from the earth to the moon, and the distance 

 between the planes of atoms in a certain crystal. Nothing 

 but confusion can result if we suppose that "length' in 

 each of these cases means the same thing, for the operations 

 employed in each case are different, and the operations 

 employed in one realm are in general meaningless if applied 

 in another. 



The result is that "if we deal with phenomena outside 

 the domain in which we originally defined our concepts, we 

 may find physical hindrances to performing the operations 

 of the original definition, so that the original operations 

 have to be replaced by others; . . . but we must recognize 

 in principle that in changing the operations we have really 

 changed the concept, and that to use the same name for 

 these different concepts over the entire range is dictated 

 only by considerations of convenience, which may sometimes 

 prove to have been purchased at too high a price in terms 

 of unambiguity." 2 Our procedure, when we find ourselves 

 confronted with a new range of phenomena, should be 

 somewhat as follows: We should "start by applying the 

 older concept until it got us into trouble; after it had got us 

 into trouble often enough we would begin to find some sort 

 of regularity in the way in which the trouble occurred, just 

 exactly like the rat trying to get out of a maze and running 



1 Ibid., p. 5 (italics are the author's). 2 Ibid., p. 23. 



