Principia Mathematica, Volume 3

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Page 233 - ... a class of one-one relations all having the same converse domain, and all having their domain contained in their converse domain. In such a case as spatial distances, the applicability of this view is obvious ; in such a case as masses, the view becomes applicable by considering eg one gramme as + one gramme, ie as the relation of a mass m to a mass m' when m exceeds m' by one gramme. What is commonly called simply one gramme will then be the mass which has the relation + one gramme to the zero...
Page v - We have developed a theory of ratios and real numbers which is prior to our theory of measurement, and yet is not purely arithmetical, ie does not treat ratios as mere couples of integers, but as relations between actual quantities such as two distances or two periods of time.

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