This is a big topic, but to say a little something about it: Essentially it means that anything can be justified with the intellect. Thoughts matter little. How you feel about those thoughts is what matters.
As you evolve, the entire process becomes more and more refined with less and less conditioning.
The best a person can do is to reflect thoughtfully and feel within themself honestly and sincerely. Then make a decision and act accordingly. Then see what happens and evaluate reflectively again, adjusting accordingly.