  A related problem for a Millian view, which arises even for the mathematics of Mill's day, is a dilemma concerning the existence of aggregates of aggregates, aggregates of aggregates of aggregates…. Higher-order aggregates, if they exist, can only be abstract—what else? But if they do not exist—if there are only first-order aggregates—then in particular there are no numbers of numbers, for instance it is meaningless or false to say that there are two primes between 20 and 30.