Money is a concept. Currency is a technology implementing that concept. This presentation will discuss the technological trade-offs made in commodity and fiat currencies, with respect to John Nash's concept of ideal money (ideal money neither inflates nor deflates) and Gresham's Law (bad money drives out good). Cryptocurrencies using Proof of Work and Proof of Stake are a specific case of commodity currency that come with their own tension between scalability, security, and decentralization, which make it difficult to realize their potential. They also come with unique vulnerabilities due real-world economies of scale, which drive the system towards real-world centralization over the long run even though from a purely digital perspective they are operating correctly. These must be addressed if they are to remain viable in the long term.