You can also be too strained. Like my teacher used to say, a cat can be extremely concentrated at a mouse hole, and all she's thinking about is getting that juicy mouse. But from a Buddhist point of view, that's not very skillful.

In the Buddhist tradition, we're also taught that concentration needs to be balanced with energy. When Joseph, Jack Kornfield (a Buddhist teacher, author, and co-founder of the Insight Meditation Society), and I first went to Barre, Mass., 25 years ago to look at a possible site for a retreat center, we visited the town square.

A cat can be extremely concentrated at a mouse hole. But from a Buddhist point of view, that's not very skillful.

It was a classic New England scene, with a town green and a statue of the founding father in the center. The Barre town motto was inscribed on the statue, and it read, "Tranquil and Alert." Well, we thought, any town that has a motto like that should have a meditation center! It's the perfect description of that balance. You can get very tranquil and be sluggish, but in meditative concentration, you are tranquil and get alert.

A good way to begin cultivating concentration is to choose an object like the breath, or phrases of lovingkindness, or a sacred word or image. Developing concentration is learning how to connect in the moment with that object and repeatedly letting go of distractions to begin again gently. An image I found helpful in my own practice was the notion of holding something very precious in my hand that if I grabbed too tightly would shatter, and that if I held too loosely would fall and break. The correct relationship would be to cherish it and stay connected to it. As we practice, breath by breath, moment by moment, the mind will go off and do its thing. It'll be scattered or restless, and we have to gently let go and come back to cherish the object of concentration a million times. That's the training of the mind.