Text 42] The Advent of Lord Krṣṇa: Introduction
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body turns totally into matter);  guṇeṣu —(the mind, not being liberated, becomes attached) to the material qualities;  māyā-raciteṣu —where the material energy creates a similar body;  dehī —the spirit soul who accepts such a body;  asau —he;  prapadyamānaḥ —being surrendered (to such a condition);  saha —with;  tena —a similar body;  jāyate —takes birth.
TRANSLATION
At the time of death, according to the thinking, feeling and willing of the mind, which is involved in fruitive activities, one receives a particular body. In other words, the body develops according to the activities of the mind. Changes of body are due to the flickering of the mind, for otherwise the soul could remain in its original, spiritual body.
PURPORT
One can very easily understand that the mind is constantly flickering, changing in the quality of its thinking, feeling and willing. This is explained by Arjuna in  Bhagavad-gītā  (6.34):
cañcalaṁ hi manaḥ krṣṇa pramāthi balavad drḍham tasyāhaṁ nigrahaṁ manye vāyor iva suduṣkaram
The mind is  cañcala , flickering, and it changes very strongly. Therefore Arjuna admitted that controlling the mind is not at all possible; this would be as difficult as controlling the wind. For example, if one were in a boat moving according to the wind on a river or the sea, and the wind were uncontrollable, the tilting boat would be very much disturbed and extremely difficult to control. It might even capsize. Therefore, in the  bhava-samudra , the ocean of mental speculation and transmigration to different types of bodies, one must first control the mind.
By regulative practice one can control the mind, and this is the purpose of the  yoga  system ( abhyāsa-yoga-yuktena ). But there is a chance of failure with the  yoga  system, especially in this age of Kali, because the  yoga  system uses artificial means. If the mind is engaged in  bhakti-yoga ,