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 MaharshiHow to get rid of the mind? Is it the mind that wants to kill itself? The mind cannot kill itself. So your business is to find the real nature of the mind. Then you will know that there is no mind. When the Self is sought, the mind is nowhere. Abiding in the Self, one need not worry about the mind.
Ramana MaharshiYou are the witness of the three bodies: the gross, the subtle, and the causal, and of the three times: past, present and future, and also this void. In the story of the tenth man, when each of them counted and thought they were only nine, each one forgetting to count himself, there is a stage when they think one is missing and do not know who it is; and that corresponds to the void. We are so accustomed to the notion that all that we see around us is permanent and that we are this body, that when all this ceases to exist we imagine and fear that we also have ceased to exist.
Ramana Maharshi