From Mindfulness-Based Cognitive Therapy: A New Approach to Preventing Relapse by Zindel V. Segal, J. Mark G. Williams & John D. Teasdale.

Because the theme of connecting to the present moment is examined in sitting meditation, participants are asked to observe in the body these reactions of aversion or attachment that arise during the practice. They are invited to notice how such reactions are powerful competitors for attention and often take awareness away from the breath, moving the focus to other, seemingly vital thoughts or feelings. The practice of mindfulness can be a powerful ally, allowing us to notice when this has occurred and to regain the ability to choose where we wish to place our attention in this moment. Note, again, that the aim of the practice is not relaxation or even happiness. Rather, it is freedom from the tendency to get drawn into automatic reactions to pleasant and unpleasant thoughts, feelings, and events. But getting drawn into such thoughts and imaginings still happens - even for experienced meditators. The promise of mindfulness practice is not that such mind wanderings will be prevented, but that a person will come to find it possible to extricate him- or herself from it in a non-judgmental way when it does occur.