SB 5.11.5
sa vasanatma visayoparakto
guna-pravaho vikrtah sodasatma
bibhrat prthan-namabhi rupa-bhedam
antar-bahistvam ca purais tanoti
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Because the mind is absorbed in desires for pious and impious activities, it is naturally subjected to the transformations of lust and anger. In this way, it becomes attracted to material sense enjoyment. In other words, the mind is conducted by the modes of goodness, passion and ignorance. There are eleven senses and five material elements, and out of these sixteen items, the mind is the chief. Therefore the mind brings about birth in different types of bodies among demigods, human beings, animals and birds. When the mind is situated in a higher or lower position, it accepts a higher or lower material body.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Transmigration among the 8,400,000 species is due to the mind’s being polluted by certain material qualities. Due to the mind, the soul is subjected to pious and impious activities. The continuation of material existence is like the waves of material nature. In this regard, Srila Bhaktivinoda Thakura says, mayara vase, yaccha bhese’, khaccha habudubu, bhai: “My dear brother, the spirit soul is completely under the control of maya, and you are being carried away by its waves.” This is also confirmed in Bhagavad-gita:
prakrteh kriyamanani
gunaih karmani sarvasah
ahankara-vimudhatma
kartaham iti manyate
“The bewildered spirit soul, under the influence of the three modes of material nature, thinks himself the doer of activities, which are in actuality carried out by nature.”
Material existence means being fully controlled by material nature. The mind is the center for accepting the dictations of material nature. In this way the living entity is carried away in different types of bodies continuously, millennium after millennium.
krsna bhuli’ sei jiva anadi-bahirmukha
ataeva maya tare deya samsara-duhkha
(Caitanya-caritamrta, Madhya 20.117)
Due to the living entity’s forgetfulness of Krsna, one is bound by the laws of material nature.