It is Atman alone that gives light to the prime faculty of our internal being called buddhi (intellect). Buddhi has three main functions: discrimination, judgement, and decision.
In Western psychology these three qualities of buddhi are considered to be aspects of the ego. In other words they are intimately connected with one's limited, subjective sense of I; it is "I" who discriminates, judges, and decides.
Eastern psychology, however, shows that these three functions of the intellect are distinct from the ego. That is, they are not necessarily connected with one's limited, subjective sense of I. In this view the powers of discrimination, judgement, and decision need not be expressed within the more circumscribed perspective of one's ego.
No action can be performed without the help of the mind; all the actions and speech of a human being are governed by the mind. Therefore that which needs the most attention in developing a practical way of living is our mental life. Training the mind means making the mind free from all complexes, and in sadhana all the faculties of mind are trained in a unified way. This is important because one-sided progress is exactly like knowing half a truth, which is not truth at all.