Vaisesika Sūtra 5.2.17

अपसर्पणमुपसर्पणमशितपीतसंयोगाः कार्यान्तरसंयोगाश्चेत्यदृष्टकारितानि ॥१७॥

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