Everyone acts under the dictation of the ego. Therefore Lord Śiva is trying to purify false egotism through the mercy of the Supreme Personality of Godhead. Since Lord Śiva, or Rudra, is himself the controller of egotism, he indirectly wants to be purified by the mercy of the Lord so that his real egotism can be awakened. Of course, Lord Rudra is always spiritually awake, but for our benefit he is praying in this way. For the impersonalist, pure egotism is ahaṁ brahmāsmi — “I am not this body; I am spirit soul.” But in its actual position, the spirit soul has devotional activities to perform. Therefore Lord Śiva prays to be engaged both in mind and in action in the devotional service of the Supreme Lord according to the direction of the Vedas. This is the process for purifying false egotism. Cetaḥ means “knowledge.” Without perfect knowledge, one cannot act perfectly. The real source of knowledge is the vācaḥ, or sound vibration, given by Vedic instructions. Here the word vācaḥ, or vibration, means the Vedic vibration. The origin of creation is sound vibration, and if the sound vibration is clear and purified, perfect knowledge and perfect activities actually become manifest. This is enacted by the chanting of the mahā-mantra: Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare. Thus Lord Śiva is praying again and again for the purification of body, mind and activities through the purification of knowledge and action under the pure directions of the Vedas. Lord Śiva prays to the Supreme Personality of Godhead so that his mind, senses and words will all turn toward devotional activities only.