Early in the book, the author writes about his encounters with Zen Master Seung Sahn, and the wisdom he imparted to his students.
I looked him up online, and came across this quote about the nature of mind:
“Our mind is like a clear glass of water.
If you put salt into the water, it becomes salt water;
sugar, it becomes sugar water;
shit, it becomes shit water.
But originally the water is clear.
No thinking, no mind.
No mind, no problem.”
-Zen Master Seung Sahn
The quote speaks for itself, and tells us that the energetic phenomenon we recognize as "mind" is intrinsically empty. We can get a glimpse of this when we focus on our sensory experiences, and let the chatter of our internal dialogue quiet down. There is "nothing" there of solidity, it is simply energy patterns bouncing around in our heads, and we can learn to direct this energy through meditation & simple mindfulness.
Whatever our attention turns to, internally or externally, is what will fill the clarity of the pure essence of the mind. This is why it is so important to practice mindfulness and directing our thoughts and actions toward positive, beneficial states of being if we do not want to be overcome by suffering.
By seeing the mind as a clear vessel, we discover that we can shape our thoughts and experiences to create the life that we wish to lead.