In the material world the activities of the mind are acceptance and rejection. As long as the mind is in material consciousness, it must be forcibly trained to accept meditation on the Supreme Personality of Godhead, but when one is actually elevated to loving the Supreme Lord, the mind is automatically absorbed in thought of the Lord. In such a position a yogī has no other thought than to serve the Lord. This dovetailing of the mind with the desires of the Supreme Personality of Godhead is called nirvāṇa, or making the mind one with the Supreme Lord.
The best example of nirvāṇa is cited in Bhagavad-gītā. In the beginning the mind of Arjuna deviated from Kṛṣṇa's. Kṛṣṇa wanted Arjuna to fight, but Arjuna did not want to, so there was disagreement. But after hearing Bhagavad-gītā from the Supreme Personality of Godhead, Arjuna dovetailed his mind with Kṛṣṇa's desire. This is called oneness. This oneness, however, did not cause Arjuna and Kṛṣṇa to lose their individualities. The Māyāvādī philosophers cannot understand this. They think that oneness necessitates loss of individuality. Actually, however, we find in Bhagavad-gītā that individuality is not lost. When the mind is completely purified in love of Godhead, the mind becomes the mind of the Supreme Personality of Godhead. The mind at that time does not act separately, nor does it act without inspiration to fulfill the desire of the Lord. The individual liberated soul has no other activity. pratinivṛtta-guṇa-pravāhaḥ. In the conditioned state the mind is always engaged in activity impelled by the three modes of the material world, but in the transcendental stage, the material modes cannot disturb the mind of the devotee. The devotee has no other concern than to satisfy the desires of the Lord. That is the highest stage of perfection, called nirvāṇa or nirvāṇa-mukti. At this stage the mind becomes completely free from material desire.
Yathārciḥ. Arciḥ means "flame." When a lamp is broken or the oil is finished, we see that the flame of the lamp goes out. But according to scientific understanding, the flame is not extinguished; it is conserved. This is conservation of energy. Similarly, when the mind stops functioning on the material platform, it is conserved in the activities of the Supreme Lord. The Māyāvādī philosophers' conception of cessation of the functions of the mind is explained here: cessation of the mental functions means cessation of activities conducted under the influence of the three modes of material nature.