यं हि न व्यथयन्त्येते पुरुषं पुरुषर्षभ। समदुःखसुखं धीरं सोऽमृतत्वाय कल्पते ॥

yaṁ hi na vyathayantyete puruṣhaṁ puruṣharṣhabha sama-duḥkha-sukhaṁ dhīraṁ so ’mṛitatvāya kalpate

The man whom these torment not, O chief of men, balanced in pain and pleasure, steadfast, he is fitted for immortality.

In plain words: A person who is not shaken by pain or pleasure stays steady and calm in both. That person is ready for eternal life.