joshi tm: You are correct, you never do know exactly what your opponent has in their hand.

It could be a well timed bluff, and every so often, you will rightly fold to a worse hand than your own.

But again, over time, a first position raise (on a full table especially) is NOT going to be a pure bluff.

...also, good poker players have learned to always mix up their play style or strategy, so good instincts and the ability to read tells becomes very important as well (some people prefer online play just because tells dont become a factor)