Vaisesika Sūtra 5.2.17

apasarpaṇam upasarpaṇam aśitapītasaṁyōgāḥ kāryāntarasaṁyōgāś cētyadṛṣṭakāritāni ॥17॥

Withdrawal, approach, eating-drinking combinations, and other functional conjunctions are caused by the unseen-force.