Reading Asset Pricing by John Cochrane (2005), in his second chapter he defines the risk free rate as:
Rf = 1 / sum [pc(s)]
Where pc(s) are state contingent claims, where s is the state of nature realised from a possible set S.
This is quite an elementary question I'm sure, but I just cannot grasp why this is the case. If anyone could enlighten me intuitively that would be most appreciated!