Mind is the nucleus of the material body. The gradual evolutionary progress of the material body depends on psychological changes within the mind. The change of the bodily construction of a worm into that of a butterfly and, in modern medical science, the conversion of a man's body into that of a woman (or vice versa) are more or less dependent on psychological changes.
In the Bhagavad-gītā it is said that if a man, at the time of death, concentrates his mind upon the form of the Personality of Godhead, Śrī Kṛṣṇa, and while so doing relinquishes his body, he at once enters the spiritual existence of the anti-material world. This means that anyone who trains the mind to turn from matter to the spiritual form of the Godhead by performance of the prescribed rules of devotional service can easily attain the kingdom of God, in the anti-material sky. And of this there is no doubt.