Further to the above comments, here is an indication of Thanissaro Bhikkhu's perspective on the matter...
When I first went to stay with Ajaan Fuang, one of the questions I asked him was, "What do you need to believe in order to meditate?" He answered that there was only one thing: the principle of kamma. Now when we hear the word "kamma," we usually think, "kamma-and-rebirth," but he meant specifically the principle of action: that what you do shapes your experience.
If you're convinced of this, you can do the meditation because, after all, the meditation is a doing. You're not just sitting here, biding your time, waiting for the accident of Awakening to happen. Even in very still states of meditation, there's an activity going on. Even the act of "being the knowing" is still a doing. It's a fabrication, a sankhara. In one of the suttas, the Buddha says that all the different khandhas, all the different aggregates that make up experience as a whole, have to get shaped into aggregates by the process of fabrication. In other words, there's a potential for a form, a potential for a feeling, potential for perception, fabrication, consciousness; and the act of fabricating is what turns these potentials into actual aggregates.
It sounds abstract, but it's a very important lesson for the meditation even from the very beginning. You sit here in the body — and of course, that's a fabrication right there: the idea that you're sitting in the body — but given all the many different things you could focus on right now, there's the possibility of choice. This possibility of choice is where kamma comes in. You can choose any of the sensations that are coming into your awareness. It's as if there were a buzz in all the different parts of the body. There's a potential for pain here, a potential for pleasure over there. All these different sensations are presenting themselves to you for you to do something about them, and you have the choice as to which ones you'll notice.
Doctors have done studies showing that pain isn't just a physical phenomenon. It isn't totally a given. There are so many different messages coming into your brain right now that you can't possibly process them all, so you choose to focus on just some of them. And the mind has a tendency to focus on pain because it's usually a warning signal. But we don't have to focus there. In other words, there can be a slight discomfort in a part of the body, and you can focus on it and make it more and more intense, more and more of an issue. That's one thing you can do right now, but — even if you may not realize it — you have the choice of whether or not to do that. You can choose not to make it more intense. You can choose even to ignore it entirely. Many times we have habitual ways of relating to sensations, and they're so habitual and so consistent that we think there's no choice at all. "This is the way things have to be," we think, but they don't.
That's the other implication of the principle of kamma: You can change your actions. If some parts of experience are dependent on choice and fabrication, you can choose to change. You see this really clearly when you focus on the breath. The breath is always there in the body, and if you look carefully you'll discover that it has many levels. It's like looking up in the sky: Sometimes you feel a breeze coming from the south, but you look up in the sky and see a layer of clouds moving east, and another higher layer of clouds moving west. There are lots of different layers of wind in the atmosphere and, in the same way, there are lots of different layers of breath in the body. You can choose which ones to focus on.
It's like having a radio receiver: You can choose to tune-in to different stations. The radio waves from all the nearby radio stations, all the different frequencies, are all in the air around us. There are radio waves from Los Angeles, radio waves from San Diego, even short wave radio waves from who-knows-where, all over the place. They're going through this room right now. They're going through your body right now. And when you turn on the radio you choose which frequency you want to focus on, which one you want to listen to. The same with the body. You sort out, of all the possible sensations, just one type of sensation to focus on: the breath-ness of the breath. Wherever you feel the sensation of the in-and-out breath most clearly, you focus right there. Now some of us have a radio we haven't taken very good care of, and as soon as we tune it in to one station it slips over to another. So you've got to keep tuning it back, tuning it back.
Source: http://www.accesstoinsight.org/lib/auth ... tml#tuning