by steady and continuous investigation

By steady and continuous investigation into the nature of the mind, the mind is transformed into That to which `I' refers; and that is in fact the Self. The mind has necessarily to depend for its existence on something gross; it never subsists by itself. It is the mind that is otherwise called the subtle body, ego, jiva or soul.

That which arises in the physical body as `I' is the mind. If one enquires whence the `I'-thought arises in the body in the first instance, it will be found that it is from the hrdayam or the Heart. That is the source and stay of the mind. Or again, even
if one merely continuously repeats to oneself inwardly `I-I' with the entire mind fixed thereon, that also leads to the same source.

~ Sri Ramana Maharshi, The Teachings of Bhagavan in His Own Words