15.20  Iti  etc.  The most secret [scripture] : Because it explains  the oneness of all.  One becomes a man of wisdom by knowing this only and not by the knowledge of worldly affairs. One becomes a man of success by just understanding this, and not even by the deeds like the total victory over the foes, earning  wealth, enjoying women and so on.  The word ca  'also'  indicates a wonder.  Has it not been witnessed that  [always] one becomes a man of  success by what has been accomplished ?  But it is strange that [in the present case one becomes a man of success] by just what has been realised.
 
The word iti  'thus'  indicates the conclusion of the treatise.  For, what is to be taught has come to an end completely.  That is why in the Sixteenth Chapter the eligibility of the pupil, Arjuna, is exclusively dealt with; and nothing new is taught.  The intention  [of that chapter] is to say only this much :  'The divine wealth is just of that nature;  but the devilish wealth born of illusion is of this nature; you (Arjuna) are however endowed with the divine wealth of wisdom'.  Hence [the Lord] is going to say  'Don't  worry.  [You are endowed] with the divine wealth'  (XVI, 5).  That is why earlier in the context of explaining the clash between the wisdom and ignorance this has been indicated [by me (Ag.)]:  'It is the confrontation between the wisdom and ignorance that has been detailed under the pretext of [describing] the wars between the  gods and devils.'  So, while dealing essentially with the ality of a pupil, other subjects are mentioned incidentally.  So also the pair of chapters (Ch. XVII &  XVIII) would follow.  But the teaching [proper] has come to an end completely here itself.  For what is to be achieved is nothing but serving (attaining) the Absolute Lord-the serving, which is of the nature of total  absorption into Him by one's entire being.  All other things are only to achieve this end.  This has been explained earlier.  The Supreme Happiness is indeed nothing but a complete absorption into the Supreme Lord by one's entire being.