Text 17

katham vidyam aham yogims
tvam sada paricintayan
kesu kesu ca bhavesu
cintyo ’si bhagavan maya

Translation

O Krishna, O supreme mystic, how shall I constantly think of You, and how shall I know You? In what various forms are You to be remembered, O Supreme Personality of Godhead? 

Commentary by Srila Prabhupada 

As it is stated in the previous chapter, the Supreme Personality of Godhead is covered by His yoga-maya. Only surrendered souls and devotees can see Him. Now Arjuna is convinced that His friend, Krishna, is the Supreme Godhead, but he wants to know the general process by which the all-pervading Lord can be understood by the common man. Common men, including the demons and atheists, cannot know Krishna, because He is guarded by His yoga-maya energy. Again, these questions are asked by Arjuna for their benefit. The superior devotee is concerned not only for his own understanding but for the understanding of all mankind. So Arjuna, out of his mercy, because he is a Vaishnava, a devotee, is opening for the common man the understanding of the all-pervasiveness of the Supreme Lord. He addresses Krishna specifically as yogin because Sri Krishna is the master of the yoga-maya energy, by which He is covered and uncovered to the common man. The common man who has no love for Krishna cannot always think of Krishna; therefore he has to think materially. Arjuna is considering the mode of thinking of the materialistic persons of this world. The words kesu kesu ca bhavesu refer to material nature (the word bhava means “physical things”). Because materialists cannot understand Krishna spiritually, they are advised to concentrate the mind on physical things and try to see how Krishna is manifested by physical representations.

Commentary by Sri Visvanatha Cakravarti Thakur

"O Lord, one who possesses the yogamaya sakti (yogin)!" This is similar to the construction of vanamalin, one who wears a garland of flowers.  How can I,  thinking of you, know you (vidyam) at all times? Of course you say bhaktya mam abhijanati yavan yas casmi tattvatah: only by bhakti can I be known as I am in truth. (BG 18.55)  But in which objects (bhavesu) can I think of you? This means "How can I perform bhakti by thinking of you in various objects?"