svayamaatmanaatmaanam vettha tvam purushottama |
bhootabhaavana bhootesha devadeva jagatpate || 15 ||

Only you yourself know of your true nature, O foremost among all, creator of all beings, lord of all beings and nourisher of this universe.
svayam : yourself
aatmanaa : of the true nature of the self
aatmaanam : by the self
vettha : know
tvam : you
purushottama : foremost among all
bhootabhaavana : creator of all beings
bhootesha : lord of all beings
devadeva : lord of all deities
jagatpate : nourisher of the universe
Previously, Arjuna said that Ishvara cannot be completely understood through our eyes and ears. But, being eager to still know Ishvara, he began using several words to describe Ishvara in this shloka. He also acknowledges that only Ishvara can know Ishvara, since there was nothing prior to Ishvara. Ishvara is self-evident, just like we do not need another source of light to see the sun.
Arjuna addressed Ishvara as “purushottama”, the foremost and eminent person, beyond all cause and effect. He is “bhootabhaavana”, the origin of all beings, the absolute reality that has taken maaya as an upaadhi or qualifier to create this world of names and forms. He is also “bhootesha”, the master and lord of all beings.
Even though he is the controller, he is not someone who is a cruel master. He is “devadeva”, the lord of all deities including Indra and Varuna, someone who is revered and adored. Also, Ishvara does not quit once the world is created. He is also “jagatpate”, the protector and nourisher of the universe. However, we need to understand that, like a magician, Ishvara is never affected by the magic show. He is the cause, and the magic show of the universe is the effect.
So, if Ishvara can alone know Ishvara, only Ishvara can reveal his glories. Arjuna takes this up next.