The Purpose of Meditation
The purpose of meditation is not to concentrate on the breath without intetrruption, forever. That by itself would be a useless goal. The purpose of meditation is not to achieve a perfectly still and serene mind. Although a lovely state, it doesn't lead to liberation by itself. The purpose of meditation is to achieve uninterrupted mindfulness. Mindfulness, and only mindfulness, produces Enlightenment.
Distractions come in all sizes, shapes, and flavors. Buddhist philosophy has organized them into categories. One of them is the category of hindrances. They are called hindrances because they block your development of both components of meditation, mindfulness and concentration. A bit of caution on this term: The word "hindrances" carries a negative connotation and indeed these are states of mind we want to eradicate. . . That does not mean, however, that they are to be repressed, avoided or condemned.
Let's use greed as an example. We wish to avoid prolonging any state of greed that arises, because a continuation of that state leads to bondage and sorrow. That does not mean we try to toss the thought out of the mind when it appears. We simply refuse to encourage it to stay. We let it come, and we let it go.
-- Henepola Gunaratana, Mindfulness in Plain English
from Everyday Mind, edited by Jean Smith, a Tricycle book