Everything in the world produces three kinds of reactions in us: pleasure, pain and indifference. Thus a painting arouses joy in the painter, pain in the observer possibly by what the picture depicts, and indifference in the salesman whose only concern is to sell it.
Going by the rule of cause and effect, what is found in the effect must be found in the cause too. That leads us to the inference that the ultimate cause of things — Prakriti here — should also be composed of the three elements, pleasure, pain and indifference. These elements are respectively sattva, rajas and tamas. These three strands of Prakriti are inseparable, though distinct. They are shining examples of unity in diversity. Another example given by Sankhya is an oil lamp. Despite being essentially different, oil, wick and flame coexist to form a lamp. Each guna struggles perpetually for preponderance over the other two. That explains the variety in creation.
The five basic elements (ether, air, fire, water and earth) called tanmatras — literally ‘that alone’ — evolve from these gunas. The mind, the vital principle called prana, five sense organs, five motor organs and the physical body are all evolutes of Prakriti, the three gunas. Thus, Prakriti, the insentient principle, is responsible for imposing the world of names and forms on the sentient principle, Purusha, by virtue of its proximity. In the undifferentiated state of Prakriti, the three gunas are in equilibrium — they fully counterbalance one another. Thus there is no rest in Prakriti. The Purusha — the Self — of course, remains unaffected by the movements and transformations in the gunas.
Our real nature is divine, the Atman, and anything other than that — our mind, sense organs, the body and the sense objects, the objects of perception by the senses — is composed of the gunas. Every one of us is a bundle of these gunas, with one of them preponderant over the other two. Thus we have three kinds of people: the calm, the active and the inert, in whom sattva, rajas and tamas respectively prevail over the other two.