Keynes was clear that his theory of interest rates is not just about money. He said it depends on three things: how much people want to keep cash, how much businesses invest, and how much people spend. This model is called the IS-LP(LM) model. It's a big deal in economics. Keynes also had another model called the D-Z model. He didn't like the old way of looking at economics that only focused on one thing at a time.