यस्त्वात्मरतिरेव स्यादात्मतृप्तश्च मानवः।आत्मन्येव च सन्तुष्टस्तस्य कार्यं न विद्यते॥
yas tvātma-ratir eva syād ātma-tṛptaś ca mānavaḥātmanyeva ca santuṣṭas tasya kāryaṃ na vidyate
Word by word
The verse says
But the one who takes his joy in himself, who is satisfied with himself and content in himself — for him there is nothing that has to be done.
In plain words
The exception. Krishna names the one person who does not need to act.
Someone who delights in the self, is satisfied by the self, content in the self. Three times, the same word.
For that person, kāryam na vidyate — there is no duty. Nothing is required.
Where you've felt this · in growing older
Someone in their eighties who has stopped needing the day to go a particular way. They still do things. They are not waiting for any of it to add up to anything.
Going deeper
Krishna has just spent six verses arguing that everyone must keep the wheel turning. Now he immediately carves out an exception, and it is worth asking why he would weaken his own case.
The answer is that he is closing an escape route before Arjuna can find it. If the exception were left unstated, Arjuna could claim it later. Krishna states it plainly and then, in verse 19, makes it irrelevant.
The threefold ātman is the key. This is 2.55 again — content in the self, by the self — and the repetition marks it as the same condition arriving from a different direction.
What exempts such a person is not merit. It is that duty runs on need, and this person needs nothing from what they do. There is no fruit they are missing, so no action is owed.
This is a real position in the Indian traditions, and the Gita acknowledges it without hesitation. What it does next is more interesting than a denial.