This is the main theme of this instruction, that tapo divyaṁ putrakā yena, yena śuddhyet sattvam. Tapo divyaṁ putra . . . putrakā yena sattvaṁ śuddhyet. Just try to purify your existence. Just like I gave you the list of purification from lowest stage of life to the highest, so, I mean to say, as the change of body, just like change of dress, you like a nice dress, giving up the old dress, similarly, change of body is going on. You accept the first-class body next life, first-class body next life which is eternal, which is not subjected to the tribulations of this body. That should be the aim of human civilization, not that simply getting money and spending for sense gratification. That is cats' and dogs' civilization. That is not human civilization. One should utilize the opportunity.
So next point is, how you can do this? How you can purify your existence? That instruction is that mahat-sevāṁ dvāram āhur vimukteḥ (SB 5.5.2). If you associate and serve mahat—mahat means the great, great personality—mahat-sevāṁ dvāram, then your door for purification will be open. Mahat-sevāṁ dvāram āhur vimuktes tamo-dvāram . . . and the opposite is, if you want to enter into the darkest region of ignorance, then you associate with persons who are simply meant for sense gratification. That's all. Two ways: this way or that way.
If you want to approach to the perfectional point of purificatory birth, then you associate with mahat-sevā. Sevā means service. Service. Our whole process is service. This spiritual life can be attained simply by service, not by any other qualification. Therefore in the Bhagavad-gītā the last instruction is surrender. You cannot render service to a person unless you surrender. That is the first condition, mahat-sevā.
Now, that surrender begins from mahat-sevā, surrendering unto a great personality. And who is great personality? That will be also explained. Generally, mahat means the great. Now, how a man can become great? When he is in contact with the great, supreme great. God is great. God is great, and if you contact God, then you become great. That is the definition given in the Bhagavad-gītā. There is no use of rubber-stamp great. That is not greatness.