The three guṇas (qualities, strands), namely sattva, rajas and tamas, belong to Nature. They strongly bind the imperishable embodied being to the body, O mighty-armed (Arjuna).
The quality or characteristic of anything is not separate from it, as are heat and brilliance from fire, flowingness from water, and coldness from ice. But these are physical or chemical properties. Guṇas, spiritual qualities, belong to Prakṛti, which cannot be without them.
In making every one undergo the effects of Prakṛti, guṇas alone are the cause. Krishna first spoke about guṇas in the 2nd chapter (2.45). In the 3rd chapter he showed their indispensability in one’s life (3.5, 27, 28, 29). In the 4th, he showed how the guṇas display a fulfilling role in preserving the society with cohesion (4.13). Again in chapter 7, he revealed how the guṇas have their overpowering note (7.12, 13, 14). In chapter 13, Krishna brings in their inevitable place and importance in conjunction with Purusha (13.20 – 22). In fact, Bhagavad Gita is an exposition of the guṇa-traya philosophy.