A Good Instrument But a Bad Master

The human vital is almost always of that nature [full of desires and fancies], but that is no reason why one should accept it as an unchangeable fact and allow a restless vital to drive one as it likes. Even apart from Yoga, in ordinary life, only those are considered to have full manhood or are likely to succeed in their life, their ideals or their undertakings who take in hand this restless vital, concentrate and control it and subject it to discipline. It is by the use of the mental will that they discipline it, compelling it to do not what it wants but what the reason or the will sees to be right or desirable. In Yoga one uses the inner will and compels the vital to submit itself to tapasya so that it may become calm, strong, obedient—or else one calls down the calm from above obliging the vital to renounce desire and become quiet and receptive. The vital is a good instrument but a bad master. If you allow it to follow its likes and dislikes, its fancies, its desires, its bad habits, it becomes your master and peace and happiness are no longer possible. It becomes not your instrument or the instrument of the Divine Shakti, but of any force of the Ignorance or even any hostile force that is able to seize and use it.

Ref : Letters on Yoga Vol. IV