Bhagavad Gita · Chapter 2: The Path of Understanding

Bhagavad Gita 2.15

yaṃ hi na vyathayantyete puruṣaṃ puruṣarṣabha | samaduḥkhasukhaṃ dhīraṃ so'mṛtatvāya kalpate ||

The person whom these do not disturb, O best of men, who is steady and the same in pain and pleasure, is fit for immortality.

Modern context

Evenness in pleasure and pain is not numbness. It is the capacity to feel both without being thrown by either. Krishna says this steadiness is what makes a person ready for the highest freedom; in ordinary life it is also what makes a person reliable.