Trading System verse Trading Psychology?

This is a favorite discussion. Many traders believe that somehow there is a system that is a "Holy Grail". I traded for a private equity fund for 15 years and never saw any trader, in any bank that I dealt with, "rely" on a "trading system". Sure some traders had their own systems to provide them with a sense of direction and trend, but they never relied on the system totally. If their system gave them an entry point, they would take it but if the price action in the market did not confirm their system signals, they would be out of the position, in a heart beat.

In addition, traders, especially in banks, have to buy and sell for their clients.  Whenever a client gives an order to the bank, it has to be executed, whether or not the bank wants to be in the counter party position. It is therefore always up to the trader to offset the position, regardless of what any system he may be using.  A trader, working for a market maker never has the luxury of only taking the position that a trading system may be suggesting. This is why there is a bid - offer spread. 

What this comment is suggesting is that a trader needs to able to execute a trade and keep himself in a winning position by relying on his mental trading abilities. Any trading system is merely a support. It's the psychology of the trader that determines his ultimate success.