In the past I never knew I had a job as border guard and I used to allow any thought to pass. The negative, unhelpful ones wrecked havoc once they settled in the country. But now I try to check the papers of each one. Take this next Parkinson's thought for example; sharp suit, greased back hair and brief case. His passport will be forged and he will promise he is here to promote "healthy" anger, defeat and mourning. There's no place for that thought.
Self fulfiling prophecy is next in line. You can tell those ideas by the way they talk to themselves; they divert energy into telling themselves not to use energy on anything; they're not good enough. They are probably the most insidious thoughts you can meet. We can't allow them to pass.
You do come across helpful thoughts; I normally wave them through. They have their papers in order and I've heard that once in the community of ideas they mobilise other thoughts and properly organise them. The most successful thoughts I've allowed to pass are self-compassion, thrownness, acceptance and living alongside a problem.
The job of border guard is a continuous, difficult one. Sometimes you misjudge a thought or are overwhelmed by the crowds at border checkpoint and the unhelpful thoughts get in. But that's ok, you can't be perfect. I've just allowed another good thought to pass.