It is a remarkable fact that only in about the last hundred years has it become apparent that the system of integers does indeed seem to have such direct physical relevance. The first example of a physical quantity which seems to be appropriately quantified by integers is electric charge.19 As far as is known (although there is as yet no complete theoretical justification of this fact), the electric charge of any discrete isolated body is indeed quantified in terms of integral multiples, positive, negative, or zero, of one particular value, namely the charge on the proton (or on the electron,
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