Bhagavad Gita Chapter 5.20 - Stable Intellect
A living entity cannot achieve real happiness by enjoying material pleasures. These pleasures are only temporary and bring duhkha, or misery. A person who is not perturbed by the occurrence of good or bad events, is steady in his intellect and is established in the eternal essence, is free from entanglements in this world. This quality is called sthira-buddhi, or self-intelligence. Na prahrsyet priyam prapya nodvijet prapya ca apriyam Whether one receives something agreeable or disagreeable in this world depends on the body he happens to be in and the karma he has accumulated through his previous life actions. So it is essential that one remains equiposed in such situations and does not become elated when he attains an agreeable situation and does not become dejected when he meets with an disagreeable situation. Such people are described as sthira-buddhir meaning steadfast in intelligence. They are not bewildered because they do not identify themselves with joy and sorrow. They