From chapter one of the book:
Lord Kṛṣṇa, the Supreme Personality of Godhead, speaks about the topmost system of yoga in the Sixth Chapter of Bhagavad-gītā. There He has explained the haṭha-yoga system. Please remember that we are preaching this Kṛṣṇa consciousness movement on the authority of Bhagavad-gītā. It is nothing manufactured. The bhakti-yoga system is authorized, and if you want to know about God, then you have to adopt this bhakti-yoga system because in the Sixth Chapter of Bhagavad-gītā it is concluded that the topmost yogī is he who is always thinking of Kṛṣṇa within himself.
Kṛṣṇa, the supreme authority, recommended the eightfold yoga system. The first step of this yoga system is to select a very secluded and sacred place. Eightfold meditation cannot be performed in a fashionable city. It is not possible. In India, therefore, those who are very serious about practicing yoga go to Hardwar, a very secluded place in the Himalayas, where they remain alone and follow a very restrictive process for eating and sleeping. There is no question of mating. Those rules and regulations must be followed very strictly. Simply to make a show of gymnastics is not perfection of yoga. Yoga means control of the senses. If you indulge your senses unrestrictedly but make a show of yoga practice, you will never be successful. You have to select a sacred place; then you have to sit with half-closed eyes and concentrate on the tip of your nose. You cannot change your posture. There are many rules and regulations which cannot possibly be followed at the present.
Even 5,000 years ago, when circumstances in the world were different, this yoga system was not practicable. Even such a great personality as Arjuna, who belonged to the royal family and was a great warrior and an intimate friend of Kṛṣṇa’s, constantly living with Him, after hearing this process of yoga from Kṛṣṇa in a face-to-face discussion, said, “My dear Kṛṣṇa, it is not possible to follow.” He flatly admitted, “For me these rules and regulations and practice for controlling the mind are not possible.” We have to think, then: 5,000 years ago a personality like Arjuna expressed his inability to practice this eightfold yoga system, so how can we follow it now?