For lack of a better description, a theory is a working set of assumptions which produces useful predictions about future events.
The best example I can come up with is how I learned to play chess.
The chess teaching set I got came with 2 sets of rules. The idea is that once you master the easier rules, it should be less difficult to move to more advanced rules.
The only major difference between the 2 sets of rules had to deal with end game.
Under the easy set of rules, you just need to capture the enemy king and you win. None of that check or checkmate, just simply take the king and you win.
We can call this the "Capture the King" theory of chess.
I can play chess using the capture the king theory, and up until the end of the game, you would have no way of telling I had a different theory of play then you.
I would still move pawns, still take your pieces and go after your king while trying to protect my king.
The capture the king theory of chess works quite well in almost every way, it just breaks down in end game.
Now realize that all theories are like this.
The theory of gravity breaks down at the subatomic and becomes unreliable. Does this mean the theory has no use?
Far from it. The theory of gravity makes many useful predictions about the natural world which prove true time and time again.
Just like with the capture the king theory, I can predict you will go after my king and protect yours. Even thought I am missing the point about why you go after my king, I can still predict that you will.
That is what makes the capture the king theory useful, I can base my play on that theory and be a competent chess player.
So just because a theory does not hold true 100% of the time, it does not mean it has no use or value.
We need to realize that a theory is only a useful set of assumptions for any given problem, and not the be all and end all of reality.
So when I hear people rally against the theory of evolution because it fails to explain the origin of life, I could only liken it to a beginner chess player throwing out the whole training course because it uses the Capture the King theory.
