Quote from rolextrader:
I define an edge as a statistical probability that will work in your favor. The analogy between a gambler going against a casino which has a mathematical certainty against the gambler on each and every decision and trader who has an edge in the market is not apples to apples. Why? Probabilities (trading edge) and certainties (casino expectation) are not one in the same.
No doubt, if a trader has no statistical probabilities working in his favor, it doesn't matter if he is like a mindless robot--it won't matter, he will lose; but having this edge does not guarantee he will be long term profitable if he has no discipline. In addition, trading edges come and go as more and more participants come into the market and compete.
The casino needs no discipline--its positive expectancy will never change as long as the current game rules/payout remain the same. All things being equal, a gambler who continually plays in a casino will eventually lose his entire bankroll despite intermittent runs of good luck.
I hope that clears this issue up.