  We suggest an underlying mechanism that governs the growth of a network of
concepts, a complex network that reflects the connections between different
scientific concepts based on their co-occurrences in publications. To this end,
we perform empirical analysis of a network of concepts based on the preprints
in physics submitted to the arXiv.org. We calculate the network characteristics
and show that they cannot follow as a result of several simple commonly used
network growth models. In turn, we suggest that a simultaneous account of two
factors, i.e., growth by blocks and preferential selection, gives an
explanation of empirically observed properties of the concepts network.
Moreover, the observed structure emerges as a synergistic effect of these both
factors: each of them alone does not lead to a satisfactory picture.
