  Invention has been commonly conceptualized as a search over a space of
combinatorial possibilities. Despite the existence of a rich literature,
spanning a variety of disciplines, elaborating on the recombinant nature of
invention, we lack a formal and quantitative characterization of the
combinatorial process underpinning inventive activity. Here we utilize U.S.
patent records dating from 1790 to 2010 to formally characterize the invention
as a combinatorial process. To do this we treat patented inventions as carriers
of technologies and avail ourselves of the elaborate system of technology codes
used by the U.S. Patent Office to classify the technologies responsible for an
invention's novelty. We find that the combinatorial inventive process exhibits
an invariant rate of "exploitation" (refinements of existing combinations of
technologies) and "exploration" (the development of new technological
combinations). This combinatorial dynamic contrasts sharply with the creation
of new technological capabilities -- the building blocks to be combined --
which has significantly slowed down. We also find that notwithstanding the very
reduced rate at which new technologies are introduced, the generation of novel
technological combinations engenders a practically infinite space of
technological configurations.
