Question : What is the nature of the mind? Ramana Maharshi : The mind is nothing other than the `I’-thought. 

The mind and the ego are one and the same. The other mental faculties such as the intellect and the memory are only this. Mind [manas], intellect [buddhi], the storehouse of mental tendencies [chittam], and ego [ahamkara]; all these are only the one mind itself. This is like different names being given to a man according to his different functions. The individual soul [jiva] is nothing but this soul or ego.

Question : How shall we discover the nature of the mind, that is, its ultimate cause, or the noumenon of which it is a manifestation?

Ramana Maharshi: Arranging thoughts in the order of value, the `I’-thought is the all-important thought. Personality-idea or thought is also the root or the stem of all other thoughts, since each idea or thought arises only as someone’s thought and is not known to exist independently of the ego. The ego therefore exhibits thought activity. The second and the third persons [he, you, that, etc.] do not appear except to the first person [I]. Therefore they arise only after the first person appears, so all the three persons seem to rise and sink together. Trace, then, the ultimate cause of `I’ or personality.

From where does this `I’ arise? Seek for it within; it then vanishes. This is the pursuit of wisdom. When the mind unceasingly investigates its own nature, it transpires that there is no such thing as mind. This is the direct path for all. The mind is merely thoughts. Of all thoughts the thought `I’ is the root. Therefore the mind is only the thought `I’. The birth of the `I’-thought is one’s own birth, its death is the person’s death. After the `I’-thought has arisen, the wrong identity with the body arises. Get rid of the `I’-thought. So long as `I’ is alive there is grief. When `I’ ceases to exist there is no grief.

Question : Yes, but when I take to the `I’-thought, other thoughts arise and disturb me.

Ramana Maharshi: See whose thoughts they are. They will vanish. They have their root in the single `I’-thought. Hold it and they will disappear.

Question : How can any enquiry initiated by the ego reveal its own unreality?

Ramana Maharshi: The ego’s phenomenal existence is transcended when you dive into the source from where the `I’-thought rises.

Source: from David Godman Excellent Book “Be As You are”