In practicing the Dhamma, if you don't foster a balance between concentration and discernment, you'll end up going wild in your thinking. If there's too much work at discernment, you'll go wild in your thinking. If there's too much concentration, it just stays still and undisturbed without coming to any knowledge either. So you have to keep them in balance.
Stillness has to be paired with discernment. Don't let there be too much of one or the other. Try to get them just right. That's when you'll be able to see things clearly all the way through. Otherwise, you'll stay as deluded as ever. You may want to gain discernment into too many things—and as a result, your thinking goes wild. The mind goes out of control. Some people keep wondering why discernment never arises in their practice, but when it does arise they really go off on a tangent. Their thinking goes wild, all out of bounds.
It's not easy to keep your practice on the Middle Way. If you don't use your powers of observation, it's especially hard. The mind will keep falling for things, sometimes right, sometimes wrong, because it doesn't observe what's going on. This isn't the path to letting go. It's a path that's stuck, caught up on things. If you don't know what it's stuck and caught up on, you'll remain foolish and deluded. So you have to make an effort at focused contemplation until you see clearly into inconstancy, stress, and not-self. This without a doubt is what will stop every moment of suffering and stress....
From READING THE MIND — Advice for Meditators
from the Talks of K. Khao-suan-luang (Upasika Kee Nanayon)
Translated from the Thai by Thanissaro Bhikkhu
Other Teachings by Upasika Kee Nanayon and highly recommended:
Pure and Simple: The Extraordinary Teachings of a Thai Buddhist Laywoman
An Unentangled Knowing