sukhamaatyantikam yattadbuddhigraahyamateendriyam |
vetti yatra na chaivaayam sthitashchalati tattvataha || 21 ||

That infinite joy which is comprehended by the intellect but is beyond the senses, when he experiences that state and is established in it, he does not move away from his essence.
sukham : joy
aatyantikam : infinite
yat : that
tat : that
buddhigraahyam : comprehended by the intellect
ateendriyam : beyond the senses
vetti : experiences
yatra : in that state
na : does not
cha : and
eva : ever
ayam : he
sthitaha : established
chalati : move away
tattvataha : in his essence
Previously, Shri Krishna explained that the perfected meditator taps into a source of permanent joy once he detaches the mind from the sense organs and connects it to his self. In this shloka, he elaborates on the nature of that joy. He says that this joy is infinite and is comprehended only by the intellect. Also, he says that once we are established in this joy, no external circumstance will knock us or take us away from this state.
Let us examine the nature of this joy. Shri Krishna says that it is aatyantikam or infinite. Now, the material world is an expert in giving us infinite sorrow. There are moments in our life when the degree of sorrow is low, and we tend to think that this is joy whereas in reality it is just a lower grade of sorrow. Any new object, person or situation that we encounter carries within it the seed for innumerable sorrows. But the joy that one gets from the self is infinite.
Why is the joy from the self infinite? All our worldly joys are dependent on external situations. For some of us, a perfect climate makes us happy, causing us to become sad if the climate changes. For some of us, a certain person makes us happy, so we become dependent on that person and consequently feel sad if that person leaves us. We keep creating subsets in the world: I like A, which means I don’t everything that is not A. But here’s the problem; A is finite and temporary. The joy experienced in the self is independent of all external situations that are temporary in nature. That is why it is infinite.
Another characteristic of this joy is that it is beyond the comprehension of the senses. Just like we cannot catch a satellite TV signal with a regular antenna, our senses cannot catch this joy. It is of a different wavelength altogether. It is only comprehended by our intellect, which operates at a much higher level than our mind and senses.
As an example, consider two teenagers who are at a party where everyone else is enjoying a cigarette. Both of them are offered a cigarette by their friends. The sense organs are reporting the same information to both the teenager’s intellects – that cigarette smoking is enjoyable and that all their friends are doing it. One teenager accepts the offer and takes a puff. But the other teenager has a refined intellect and it “sees” that this will only lead to sorrow in the end. In the same way, the intellect experiences joy that the senses cannot experience.
Shri Krishna further goes on to say that once the perfected meditator is established in this joy, he will never deviate from it. It is like a child learning that 2+2 = 4. Once he has internalized this teaching, it stays with him throughout his life. Similarly, once the perfected meditator realizes this self as his true nature, he will not feel the need to take on any other role or identification for the purpose of fulfillment.