Vaisesika Sūtra 10.1.01

इष्टानिष्टकारणविशेषाद् विरोधाच्च मिथः सुखदुःखयोरर्थान्तरभावः ॥१॥

iṣṭāniṣṭa-kāraṇa-viśeṣāt virodhāt ca mithaḥ sukha-duḥkhayoḥ arthāntara-bhāvaḥ ॥1॥

[इष्ट (iṣṭa) - desirable; अनिष्ट (aniṣṭa) - undesirable; कारणविशेषात् (kāraṇa-viśeṣāt) - due to distinction of causes; विरोधात् (virodhāt) - due to opposition; च (ca) - and; मिथः (mithaḥ) - mutually; सुखदुःखयोः (sukha-duḥkhayoḥ) - of pleasure and pain; अर्थान्तरभावः (arthāntara-bhāvaḥ) - difference in nature;]

Due to distinct causes of desirable and undesirable and their opposition, pleasure and pain are of a different nature.