# Why is the Risk Free Rate 1 over Contingent Claim Prices?

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!