After elaboration of karma-yoga in chapter three, now in chapter four Lord Krsna elaborates on knowledge of ātmā. In chapter two itself ātmā is explained to be unperceivable nor can be thought of, and also in Mundaka Upanishad 1.1.6 says:
yattadadreśyamagrāhyamagotramavarṇamacakṣuḥśrotraṃ tadapāṇipādam | nityaṃ vibhuṃ sarvagataṃ susūkṣmaṃ tadavyayaṃ yadbhūtayoniṃ paripaśyanti dhīrāḥ ||
That which cannot be perceived, which cannot be seized, which has no origin, which has no properties, which has neither ear nor eye, which lias neither hands nor feet, which is eternal, diversely manifested, all-pervading, extremely subtle, and undecaying, which the intelligent cognized as the source of the Bhutas.
Then how this knowledge could be understood by us? Experience of pure ātmā is impossible, but every experience itself is ātmā alone. Therefore Lord Krsna explains the nature of ātmā through the nature of action where the existence of ātmā is wrongly taken to be the doer of action. When we want to look for a sword, we should look for its cover first, is it not? Similarly by understanding the nature of action and inaction, we come to understand ātmā and non-ātmā.
karmaṇyakarma yaḥ paśyedakarmaṇi ca karma yaḥ|
sa buddhimān manuṣyeṣu sa yuktaḥ kṛtsnakarmakṛt ||4.18||
The one who sees actionlessness in action and action in actionlessness is wise among human beings. The person is a yogī, who has done everything that is to be done.
This is one of the highly confusing verse in gītā. As our understanding, karma is doing action and akarma is not doing action. How come the wise sees actionlessness in action and action in actionlessness?
What is the meaning of "action in actionlessness". Actionlessness as our usual understanding is absent of action, but from verse 3-5, we come to know that it is impossible for a person to be actionless /absent of action, therefore the meaning of actionlessness as absent of action is ruled out. Because when we decided not to do any action, the doership for this decision must be there. If I decide not to walk, I need to engage myself in the activity of standing or sitting. If I decide not to move, I need to engage myself in the activity of stillness. Being still it self is a difficult action. Doership is the based for all actions, therefore a wise person who understand the nature of karma can see this fact that action in actionlessness.
How about "actionlessness in action"? Action is activity or movement, which is based on desire produced by the mind. Because of having desire to move the book, then hand is engaged to lift the book. This action of lifting involved hand, eyes, mind, and the doer who identifies with this body-mind-sense-complex, which called ahamkāra, which is not my real nature. Remember that anything which can be negated is not my true nature. Since this identification dropped during deep sleep, therefore ahamkāra is not my real nature. My real nature is sat-cit-ānanda-ātmā, being exist, conscious and limitlessly present in every being. Because of my mere present, the body-mind-sense-complex become sentient, hence activities can be done. But I am not the doer of those activities. Just like by the mere present of the sun, the ray help photosynthesis of plants, can turns to electricity through the solar system, and even can burn the forest. But can we say that the sun is the doer of all these activities? Does the sun has the will or doership in all these activities? No, heat is the nature of the sun, just like being conscious is the nature of ātmā, and lending the consciousness to body-mind-sense-complex is natural. Therefore I am ātmā is actionless in action.
Ātmā being limitless is all pervasive. There is no place where ātmā is absent. Being all pervasive, movement is impossible. Because movement for a particular entity is possible only when there is absent of this entity. This body can move from here to the kitchen because there is absent of this body in the kitchen, otherwise movement is impossible. When there is no movement, there is no action. Therefore ātmā being all pervasive / limitless is actionless.
One who knows this fact is kṛtsnakarmakṛt - who has achieved everything that is to be achieved. We are only achieving what is not yet to be achieved. What is not yet to be achieved by a person is limiting him/her. Therefore one who has achieved everything is one who is free from all limitation. One who know "I am ānanda - limitless ātmā" is kṛtsnakarmakṛt.