It is one thing to develop a trading edge; quite another to follow through with it. On a rational level setting up a trading edge based on historical testing, with a pre-determined risk calculation (i.e., where to exit if wrong) and how long to stay in if right, is common sense, especially since the future is unknown. The only way to plan for an unknown future is by planning for several possible scenarios. This is rational. To assume that the future can be pre-determined, that we can predict the buying and selling decisions of all other traders and investors is a pipedream. It is purely irrational. It is indeed foolish.