The increasing complexity of subjective mathematical knowledge can also be attributed to horizontal processes of concept and property elaboration and clarification.
This horizontal process of object formation in mathematics is that described by Lakatos (1976), in his reconstruction of the evolution of the Euler formula and its justification.
Namely, the reformulation (and ‘stretching’) of mathematical concepts or definitions to achieve consistency and coherence in their relationships within a broader context.
This is essentially a process of elaboration and refinement, unlike the vertical process which lies behind ‘objectification’ or ‘reification’.