The document discusses the concept of interest and how it relates to the financial system, even in a hypothetical scenario without money. It describes how savings, investment, and lending can occur through the exchange of goods and services, using the example of castaways on a deserted island. One castaway invests time to make a shovel, allowing greater future food production, while another borrows the shovel but must provide compensation due to risks like default or damage to the capital good. This hypothetical scenario parallels how interest, risk, and time preference work in a modern financial system.