The culprit in this story, the element that throws the entire system out of whack, is private investment. Private investment consists of business expenditures on machines, buildings, factories, and so on. In other words, investment is capital formation. Keynes claimed that private investment is inherently unstable due to what he called the “animal spirits” of businessmen/capitalists. He believed that businessmen are ultimately irrational and prone to herd-like behavior. Like sheep that blindly follow other sheep in the herd, it is easy for businessmen to become “irrationally exuberant”—as well as irrationally lethargic. Investment lethargy would trigger a large decrease in private investment, thus decreasing aggregate expenditures and triggering an economic downturn.