Pure devotional service must be freed from all desires for sense gratification, fruitive aspirations and culture of monistic knowledge. One must be freed from all kinds of designations, and when one is thus converted to transcendental purity, one can then serve the Lord by purified senses.
As long as there is the desire to enjoy sensually or to become one with the Supreme or to possess the mystic powers, there is no question of attaining the stage of pure devotional service.
Devotional service is conducted under two categories, namely primary practice and spontaneous emotion. When one can rise to the platform of spontaneous emotion, he can make further progress by spiritual attachment, feeling, love, and many higher stages of devotional life for which there are no English words. We have tried to explain the science of devotional service in our book The Nectar of Devotion, based on the authority of Bhakti-rasāmṛta-sindhu by Śrīla Rūpa Gosvāmī.