When you practice Self-Inquiry and follow the thoughts back to its source, the whole secret is to realize, there is no source to your thoughts.
It is only the source that exist, but there is no source for your thoughts, for your body, for your mind.
That has no source.
For Brahman is self-contained absolute reality.
There are no thoughts in Brahman.
Brahman is not a source.
Brahman is something beyond words and thoughts.
Something that you alone can experience.
In other words Brahman can be experienced.
Not as anything you're aware of now, not as a person, place or thing but as life experiences itself.
Ponder these things deeply.
232 The Ground Upon Which I Stand Is Holy Ground - March 18, 1993