SB 6.1.51

tad etat ṣoḍaśa-kalaṁ
 liṅgaṁ śakti-trayaṁ mahat
dhatte ’nusaṁsṛtiṁ puṁsi
tat — therefore; etat — this; ṣoḍaśa-kalam — made of sixteen parts (namely the ten senses, the mind and the five sense objects); liṅgam — the subtle body; śakti-trayam — the effect of the three modes of material nature; mahat — insurmountable; dhatte — gives; anusaṁsṛtim — almost perpetual rotation and transmigration in different types of bodies; puṁsi — unto the living entity; harṣa — jubilation; śoka — lamentation; bhaya — fear; ārti — misery; dām — which gives.
The subtle body is endowed with sixteen parts — the five knowledge-acquiring senses, the five working senses, the five objects of sense gratification, and the mind. This subtle body is an effect of the three modes of material nature. It is composed of insurmountably strong desires, and therefore it causes the living entity to transmigrate from one body to another in human life, animal life and life as a demigod. When the living entity gets the body of a demigod, he is certainly very jubilant, when he gets a human body he is always in lamentation, and when he gets the body of an animal, he is always afraid. In all conditions, however, he is actually miserable. His miserable condition is called saṁsṛti, or transmigration in material life.

The sum and substance of material conditional life is explained in this verse. The living entity, the seventeenth element, is struggling alone, life after life. This struggle is called saṁsṛti, or material conditional life. In Bhagavad-gītā it is said that the force of material nature is insurmountably strong (daivī hy eṣā guṇa-mayī mama māyā duratyayā). Material nature harasses the living entity in different bodies, but if the living entity surrenders to the Supreme Personality of Godhead, he becomes free from this entanglement, as confirmed in Bhagavad-gītā (mām eva ye prapadyante māyām etāṁ taranti te). Thus his life becomes successful.