ABSTRACT
It is one of the assumptions of educated common-sense that two magnitudes of the same kind must be numerically comparable. People are apt to say that they are thirty per cent, healthier or happier than they were, without any suspicion that such phrases are destitute of meaning. The purpose of the present chapter is to explain what is meant by measurement, what are the classes of magnitudes to which it applies, and how it is applied to those classes.
