Before anything becomes a thought, it is a feeling. These are the three things: feeling comes first, then comes thought, then comes the act. You may not be aware at all that each thought is produced by a certain feeling. If the feeling is not there, the thought will not come. Feeling becomes actualized in thought, thought becomes actualized in the act.
Now you have to do the almost impossible thing -- to catch hold of a certain feeling.
Have you not watched sometimes? You don't really know why you are feeling a little disturbed; there is no real thought that can be caught as the cause, but you are disturbed, you feel disturbed. Something is getting ready underground, some feeling is gathering force. Sometimes you feel sad. There is no reason to feel sad, and there is no thought to provoke it; still the sadness is there, a generalized feeling. That means a feeling is trying to come above ground, the seed of the feeling is sending its leaves out of the ground.
If you are able to become aware of the thought, then sooner or later you will become aware of the subtle nuances of the feeling.