This paper uses the logic of double-entry bookkeeping, as it is universally used in the economic life of banks, central banks and companies, to precisely define the nature of claims and money. First, it shows that these two forms of value are always distinct in accounting, but that the claim is by nature unstable, ambivalent, and can evolve towards the monetary form under certain conditions. Then, in a second step, we show that the claim has three sources in return for which it is issued: the loan by money creation, the loan of existing money, and the forward sale of goods. These accounting relationships, which are in fact a definition of book value, then allow us to sketch out an analysis of contemporary financial and monetary crises and how to respond to them effectively.