 Russell (1903: ch. 26) also had a key role in leading philosophers to acknowledge that at least some relations, in particular spatio-temporal ones, are external, i.e., cannot be reduced to monadic properties, or the mere existence, of the relata, in contrast to internal relations that can be so reduced. This was a breakthrough after a long tradition tracing back at least to Aristotle and the Scholastics wherein there seems to be hardly any place for external relations (see entry on  medieval theories of relations).[4]