While measurement is one of the gods modern psychologists:'pay homage to with great regularity, the subject of measurement remains as elusive as ever. A systema, tic treatment of the theory is not readily found in the psychological literature. For the most part a student of the subject is confronted with an array of bewildering and conflicting catechisms, catechisms which tell him whether such and such a ritual is permissible, or, at, least, whether it can be condoned. To cite just one peculiar, yet uniformly accepted ex~ ple, as elementary science students we are constantly warned that it" does not make sense"(a phrase often used when no other argument is apparent) to add together numbers representing distinct properties, say, height and weight. Yet as more advanc" d physics students we are taught, with some effort no doubt, to multiply together numbers representing such things as velocity and time, or to divide distance numbers by time numbers. Why does multiplication make" more sense" than addition?