In the classical-neoclassical model and their contemporary interpretations, the free market economy is operating at full employment level of equilibrium. A. Smith’s invisible hand, and law of markets developed by J.B. Say, guarantees that economic crisis will not happen unless there is government intervention. It is suggested that a crisis will not occur due to the internal factors originating from the nature of the system. Micro-explanations of this approach assume that economic actors act rationally and make the most appropriate decisions only for their economic purposes. Rational behavior, i.e. maximizing the benefits, is possible under conditions where there is no uncertainty and the necessary information is available. Decision-makers are defined as homo economicus, expressing uniform individuals isolated from their social environment. In recent years, especially in studies in the context of behavioral economics, the assumption of rational behavior embodied by homo economicus of the orthodox approach hasbegun to be discussed. Studies such as those written by G. Akerlof and R. Shiller emphasize that non-rational instinctive and psychological factors should be taken into account to explain business cycle. These discussions brought the concept of animal spirits, which was developed in General Theory and has not been discussed for a long time, back on the agenda. According to Keynes, there is no internal mechanism that enables the free market economy to automatically achieve equilibrium associated with full employment. On the contrary, the economy tends to be underemployed-equilibrium. The most important factor that leads to this result is the deficient of effective demand based on expectations. Keynes emphasizes that the future is uncertain and explains how long-term investment decisions are made with the animal spirits concept. In an economy where rational calculations are not possible due to uncertainty, animal spirits are the main source of decision. The activities stimulated by animal spirits are irrational. The deficiency in effective demand and business cycles can be explained by the concept of animal spirits.