5.553 Russell said that there were simple relations between different numbers of things (individuals). But between which numbers? And how should this be decided? – By experience?

(There is no pre-eminent number.)

Black (p. 304) gives the reference here as this: in Logic and Knowledge p. 206 Russell writes: “I see no particular reason to suppose that the simplest relations that occur in the world are (say) of order n, but there is no a priori reason against it.”