Alan Wallace continues his commentary on how shamatha practice alone is not enough. Follow along on page 146 in your copy of the text or in the excerpt below.
Then investigate the mind as the agent that conjures up all kinds of thoughts, seeking out its shape, color, and form, as well as its source, beginning, and end, and whether it really exists or is totally nonexistent. By doing so, once you have determined with confidence that it cannot be established in any way at all, you have entered the path. [471]
If you forcefully suppress the thought process while focusing your mind single-pointedly on such things as a stick or pebble, then many obsessive thoughts will arise, as if you had blocked an irrigation canal, and your body, speech, and mind may become sharply uncomfortable. In that case, loosely relax and watch your thoughts from afar, clearly observing whatever arises.