  Citation networks of scientific publications offer fundamental insights into
the structure and development of scientific knowledge. We propose a new
measure, called intermediacy, for tracing the historical development of
scientific knowledge. Given two publications, an older and a more recent one,
intermediacy identifies publications that seem to play a major role in the
historical development from the older to the more recent publication. The
identified publications are important in connecting the older and the more
recent publication in the citation network. After providing a formal definition
of intermediacy, we study its mathematical properties. We then present two
empirical case studies, one tracing historical developments at the interface
between the community detection literature and the scientometric literature and
one examining the development of the literature on peer review. We show both
conceptually and empirically how intermediacy differs from main path analysis,
which is the most popular approach for tracing historical developments in
citation networks. Main path analysis tends to favor longer paths over shorter
ones, whereas intermediacy has the opposite tendency. Compared to main path
analysis, we conclude that intermediacy offers a more principled approach for
tracing the historical development of scientific knowledge.
