Rod Hay writes: "The Keynesians introduced the multiplier (the idea that the increase in aggregate demand would be larger than the increase in public spending)"

1. The multiplier had a history in economic theory that predates Keynes (see Hugo Hegeland, 1954, The Multiplier Theory, Lund) 

2. Richard Kahn read Pigou's Industrial Fluctuations - and presumably the detailed mathematical version of the multiplier contained therein - prior to the hiking trip on which he 'discovered' the multiplier.

Robert Leeson