If you know that the arising thought is already delusion, you are already free.How does this work in real life? Can we use the mind to determine which thoughts are true and which are false? There is a great Koan about this:
Master Ruìyán Shīyan used to call to himself every day, “Master.”This is an excellent koan because it is so easy to identify with. Especially if we think that one of our thought streams is more reliable than the other(s).
He would answer, “Yes?”
“You must keep clear.”
“Never be deceived by others, any day, any time.”
The question being "Which one is the true master?"
The responses to the question "What percentage of thoughts do you believe?" are interesting. Most people [in a small survey] asked , respond with a question like "What do you mean?"
I asked a friend of mine who is respected along the Zen path this question. His response was fairly specific - and it was below 50%.
As taught in the Dropping Ashes on the Buddha class, the thoughts are always going to be there. It is simply our attachment to them that causes us so many problems.
As a practical means, it is easy to find thoughts that turned into actions that caused harm to ourselves or others. It is easy to find false beliefs regarding what we think we want, what we think will make us happy, and even what we even think of ourselves.
So we, as Zen students, pay attention to the thoughts that arise. We see where they come from and where they go. We learn to stop buying into them. Then we may know who is the true master.