Those practicing mindfulness come to realize that "thoughts are just thoughts". One is then free to let their thoughts go because they no longer reflect reality or truth. -  What is Mindfulness? (accessed 7/6/14)

So “thoughts are just thoughts” means thoughts don’t reflect reality or truth? That they’re just in the head? I beg to differ.

Brains were designed to track the world. Brains track the world to predict probabilities of future events. Tracking and predicting guide action decisions. Thoughts are part of the brain’s tracking system. That doesn’t mean there’s a perfect fit between thought and reality. But if the brain and its tracking systems didn’t do a reasonable job of approximating reality, I wouldn’t be able to write this sentence because Homo sapiens wouldn’t even exist. An animal has to be tuned into the world in order to survive that world.

Thoughts vary in their fidelity to what is the case. Tracking is selective and prediction is guesswork. Our tracking systems may overlook something important or give too much attention to something inconsequential.  Brains aren’t omniscient. To error is human. Which is not the same as saying to be human is to error. Or to think is to be mistaken.