Concentration is not thinking of one thing. On the contrary, it is excluding all thoughts, since all thoughts obstruct the sense of one's true being. All efforts are to be directed simply to removing the veil of ignorance. Concentrating the mind solely on the Self will lead to happiness or bliss. Drawing in the thoughts, restraining them and preventing them from straying outwards is called detachment (vairagya). Fixing them in the Self is spiritual practice (sadhana). Concentrating on the heart is the same as concentrating on the Self. Heart is another name for Self.
Ramana MaharshiThe Self alone exists. When you try to trace the ego, which is the basis of the perception of the world and everything else, you find the ego does not exist at all and neither does all this creation that you see.
Ramana MaharshiThe Seeker himself becomes the knower. The thing to be known is already there. There is nothing to be known afresh. More-over there are no two things. There is only the seer, the knower.
Ramana MaharshiEvery living being longs always to be happy, untainted by sorrow; and everyone has the greatest love for himself, which is solely due to the fact that happiness is his real nature. Hence, in order to realize that inherent and untainted happiness, which indeed he daily experiences when the mind is subdued in deep sleep, it is essential that he should know himself. For obtaining such knowledge the inquiry 'Who am I?' in quest of the Self is the best means.
Ramana Maharshi