Both approaches were concerned with investment planning and both (albeit to different degrees) recognized that expanding output ahead of demand would give rise to further complementary investments and innovations. This was particularly true of infrastructure investments, given the significant indivisibilities those involved. Indeed, the two theories began w ith the challenge of a divergence between social and private returns, employed much the same conceptual framework - indivisibilities, externalities, increasing returns, complementarities in supply and demand - and acknowledged a central role for the state.