Successful trading is part science and part art. If you have found a mechanical trading system that works and you are really disciplined, you can simply apply the rules and let the mathematics work. However, if you are like most of us, you will get greedy, get scared, change your mind, decide to tweak the system, etc. In addition, the market will go against even the best of trading systems. This is where most of us get into trouble. When one recognizes that a trade is not working, even following our system, we will want to repair it.