“I am limitless Brahman” can be opposed by obstruction, because it involves the self evident me who doesn’t experience that way. These obstructions are in the form of wrong perception about myself. To remove this wrong notion we need to negate what I am not in my perception. Only when the wrong notion about myself is gone, then the correct understanding can sink in. Besides that we also need to enquire into the mithyatvam of the world which can’t affect satyam me.
The entire life, we are committing to external objects and goals to fulfil the self which we think is limited. Therefore if purusārtha - what one really seeks in life is not very well understood, then the mind will commit to variety of things and will not stay with the pursuit. Therefore objectivity - vairagyam and mental composure - samādhāna are needed. These two can be cultivated through a lifestyle of sannyāsa which is the topic for the next three verses.
buddhyā viśuddhayā yukto dhṛtyātmānaṁ niyamya ca । śabdādīn viṣayāṁstyaktvā rāgadveṣau vyudasya ca ॥18-51॥
The one who is endowed with a mind that is very clear, mastering the body-mind-sense complex with a firm resolve, giving up the sense objects such as sound, etc., and giving up likes and dislikes…
viviktasevī laghvāśī yatavākkāyamānasaḥ । dhyānayogaparo nityaṁ vairāgyaṁ samupāśritaḥ ॥ 18-52॥ …who lives in a quiet place, who eats lightly, whose speech, body and mind are mastered, who is always committed to contemplation, who has completely resorted to freedom from longing…
ahaṅkāraṁ balaṁ darpaṁ kāmaṁ krodhaṁ parigraham । vimucya nirmamaḥ śānto brahmabhūyāya kalpate ॥18-53॥ …giving up misplaced “I sense”, power, vainfullness, binding desire, anger, ownership (of external things), the one who has no sense of ownership (of his own body, etc.) and who is tranquil, is fit to gain abidance in the knowledge of Brahman being oneself.
brahmabhūyāya kalpate - one who is able to abide in the knowledge of being Brahman. What are his characteristics? buddhyā viśuddhayā yuktaḥ - one who is endowed with a very clear mind. After living a life of karma-yoga for a length of time, he no longer has any confusion about what he seeks in life. Because he is very clear about what he wants, he does not attribute to things a value which they do not have, and doesn’t get carried away by false pursuits. dhṛtyātmānaṁ niyamya - gaining mastery over the body-mind-sense complex with firm resolve. For one who recognises the purpose of life is not satisfying fancies, and has a commitment to his pursuit of moksa, the physical body-mind-sense complex becomes a vehicle for his pursuit.
Here we are talking about a life of renunciation, the life of a sannyāsī. This involves giving up pursuit of the sense objects. So, Lord Krsna says śabdādīn viṣayāṁstyaktvā - giving up pursuit of sense objects that satisfy the sense organs which are sound, touch, taste, form and scent. It doesn’t mean to abandon them, for that would mean giving up food etc. but doing only activities for the sustenance of the body, giving up likes and dislikes - rāgadveṣau vyudasya, with the mind totally dedicated to the pursuit of jñāna-nisthā.