Edge is anything that helps you make profits consistently, meaning any technique, combination of techniques, use of advanced hardware/software, math, psychology, philosophy, you name it. Michael Harris has shown mathematically that the positive expectation formula reduces to the trivial equation: Profit > 0. This means to me that this expectation formula pushed by some authors describes only a tiny subset of all possible edges. Please pay attention to this: a tiny subset of all possible edges. Traders get stuck with this trivial formula and waste their time looking for a mathematical edge when there are many other possibilities. This is the link to the original Harris' derivation:
http://bit.ly/ndeJUa
http://bit.ly/ndeJUa