The heart is what is
neutral and still. It doesn't think anything at all. It is simply aware of its
stillness. The heart is a genuinely neutral or central phenomenon. Neutral with
no past, no future, no good, no evil: That's the heart. When we talk about the
heart of anything, we mean its center. Even the human heart, which is a mental
phenomenon, we say lies in the center of the chest. But where the real heart
is, we don't know. Try focusing your attention on any part of the body, and
you'll feel the awareness of that spot. Or you can focus your attention outside
the body -- on a post or the wall of a house, for example -- and that's the
spot you'll be aware of.
So we can conclude that the true heart is still and neutral awareness. Wherever there is neutral awareness, that's where the heart is.
When people in general talk about the heart, that's not the true heart. It's simply a set of muscles and valves for pumping blood throughout the body to keep it alive. If this pump doesn't send blood throughout the body, the body can't live. It'll have to die. The same holds true with the brain. The mind thinks of good and evil by using the brain as its tool. The nervous system of the brain is a physical phenomenon. When its various causal factors are cut off, this physical phenomenon can't last. It has to stop.
But as for the mind, which is a mental phenomenon, Buddhism teaches that it continues to exist and can take birth again. This mental phenomenon will stop only when insight discerns its causal factors and uproots their underlying causes.
None of the various subjects and sciences of the world have an end point. The more you study them, the more they fan out. Only Buddhism can teach you to reach an end. In the first stage, it teaches you to acquaint yourself with your body, to see how it is made up of various things (the 32 parts) put together, and what their duties are. At the same time, Buddhism teaches you to see that the body is inherently unattractive. It teaches you to acquaint yourself with this world (the world of a human being), which is made up of suffering and stress, and which will ultimately have to fall apart by its very nature.
So now that we have received this body -- even though it is full of foul and unattractive things, and even though it is made up of all kinds of suffering and stress -- we're still able to depend on it for a while, so we should use it to do good to repay our debts to the world before we leave it at death.
The Buddha teaches that although the nature of a person (this world) is to fall apart and die, the mind -- the overseer of this world -- must come back to be reborn as long as it still has defilements. Thus he teaches us to practice concentration, which is an affair exclusively of the mind. Once we have practiced concentration, we will fell every sensory contact inside, just at the mind. We won't be concerned with out seeing and hearing at the eye or the ear. Instead, we will be aware of the sensory contact right at the mind. This is what it means to narrow down the world.
The senses are the best means for taking the measure of your own mind. When sensory contact strikes the mind, does it have an impact on you? If it has a lot of impact, that shows that your mindfulness is weak, and your foundation is still shaky. If it has only a little impact, or no impact at all, that shows that your mindfulness is strong, and you are fully able to care for yourself.
These things are like Devadatta, who created trouble for the Bodhisattva all along. If not for Devadatta, the Bodhisattva wouldn't have been able to bring his character to full perfection. Once his character had been fully perfected, he was able to gain Awakening and become the Buddha. Before gaining Awakening, he had to withstand the massive armies of temptation; and right after his Awakening, the three daughters of temptation came to test him once more. As a result, the people of the world have praised him ever since for having conquered defilement in this world once and for all.
As long as the inner senses still exist, mental contact is still a preoccupation. Thus those who know, having seen the harm of these things, are willing to withdraw from them, leaving just the heart which is neutral...neutral...neutral, with no thinking, no imagining, no fashioning of anything at all. When this is the case, where will this world be formed? This is how the Buddha teaches us to reach the world's end.