Inside money can be depend very broadly as any privately-issued long-term paper that is held by a number of agents in succession. Whenever paper circulates as a means of short-term saving (liquidity), it can properly be considered as money, or a medium of exchange, because agents hold it not for its maturity value but for its exchange value. In this article, we construct a model of an economy in which agents borrow by issuing long-term paper, and use the model to ask: When and why is circulation of the paper essential to the smooth running of the economy? If there is a shortage of liquidity, what are the symptoms? How does the economy respond?