Therefore, both the yogas are interdependent, as religion and philosophy. Religion without philosophy is sentiment, or sometimes fanaticism, while philosophy without religion is mental speculation. The ultimate goal is Kṛṣṇa, because the philosophers who are also sincerely searching after the Absolute Truth come in the end to Kṛṣṇa consciousness.
(Bhagavad-Gita-----3:3-----purport).

The word muni means one who can agitate his mind in various ways for mental speculation without coming to a factual conclusion. It is said that every muni has a different angle of vision, and unless a muni differs from other munis, he cannot be called a muni in the strict sense of the term. Nāsāv ṛṣir yasya mataṁ na bhinnam (Mahābhārata, Vana-parva 313.117). But a sthita-dhīr muni, as mentioned herein by the Lord, is different from an ordinary muni. The sthita-dhīr muni is always in Kṛṣṇa consciousness, for he has exhausted all his business of creative speculation. He is called praśānta-niḥśeṣa-mano-rathāntara (Stotra-ratna 43), or one who has surpassed the stage of mental speculations and has come to the conclusion that Lord Śrī Kṛṣṇa, or Vāsudeva, is everything (vāsudevaḥ sarvam iti sa mahātmā su-durlabhaḥ). He is called a muni fixed in mind.
(Bhagavad-Gita-----2:56-----purport).

No one can manufacture a religious principle by imperfect speculation. One must follow in the footsteps of great authorities like Brahmā, Śiva, Nārada, Manu, the Kumāras, Kapila, Prahlāda, Bhīṣma, Śukadeva Gosvāmī, Yamarāja, Janaka, and Bali Mahārāja. By mental speculation one cannot ascertain what is religion or self-realization. Therefore, out of causeless mercy to His devotees, the Lord explains directly to Arjuna what action is and what inaction is. Only action performed in Kṛṣṇa consciousness can deliver a person from the entanglement of material existence.
(Bhagavad-Gita-----4:16-----purport).

No one can be spiritually realized by manufacturing his own process, as is the fashion of the foolish pretenders. The Bhāgavatam (6.3.19) says, dharmaṁ tu sākṣād bhagavat-praṇītam: the path of religion is directly enunciated by the Lord. Therefore, mental speculation or dry arguments cannot help lead one to the right path. Nor by independent study of books of knowledge can one progress in spiritual life. One has to approach a bona fide spiritual master to receive the knowledge. Such a spiritual master should be accepted in full surrender, and one should serve the spiritual master like a menial servant, without false prestige. Satisfaction of the self-realized spiritual master is the secret of advancement in spiritual life.
(Bhagavad-Gita-----4:34-----purport).

In this Fifth Chapter of the Bhagavad-gītā, the Lord says that work in devotional service is better than dry mental speculation. Devotional service is easier than the latter because, being transcendental in nature, it frees one from reaction.
(Bhagavad-Gita-----5:1-----purport).

This Fifth Chapter is a practical explanation of Kṛṣṇa consciousness, generally known as karma-yoga. The question of mental speculation as to how karma-yoga can give liberation is answered herewith. To work in Kṛṣṇa consciousness is to work with the complete knowledge of the Lord as the predominator. Such work is not different from transcendental knowledge. Direct Kṛṣṇa consciousness is bhakti-yoga, and jñāna-yoga is a path leading to bhakti-yoga.
(Bhagavad-Gita-----5:29-----purport).

By realized knowledge, one becomes perfect. By transcendental knowledge one can remain steady in his convictions, but by mere academic knowledge one can be easily deluded and confused by apparent contradictions. It is the realized soul who is actually self-controlled, because he is surrendered to Kṛṣṇa. He is transcendental because he has nothing to do with mundane scholarship. For him mundane scholarship and mental speculation, which may be as good as gold to others, are of no greater value than pebbles or stones.
(Bhagavad-Gita-----6:8-----purport).

One should engage oneself in the practice of yoga with determination and faith and not be deviated from the path. One should abandon, without exception, all material desires born of mental speculation and thus control all the senses on all sides by the mind.
(Bhagavad-Gita-----6:24-----purport).

One cannot understand the Supreme Personality of Godhead, Kṛṣṇa, or His form, quality or name simply by mental speculation or by discussing Vedic literature. One must understand Him by devotional service. When one is fully engaged in Kṛṣṇa consciousness, beginning by chanting the mahā-mantra—Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare—then only can one understand the Supreme Personality of Godhead. Nondevotee impersonalists think that Kṛṣṇa has a body made of this material nature and that all His activities, His form and everything are māyā. These impersonalists are known as Māyāvādīs. They do not know the ultimate truth.
(Bhagavad-Gita-----7:24-----purport).

The words idaṁ viditvā indicate that one should understand the instructions given by Śrī Kṛṣṇa in this chapter and the Seventh Chapter of Bhagavad-gītā. One should try to understand these chapters not by scholarship or mental speculation but by hearing them in association with devotees.
(Bhagavad-Gita-----8:28-----purport).

In other words, persons who mock Kṛṣṇa are to be understood to be demonic or atheistic. As described in the Seventh Chapter of Bhagavad-gītā, such demonic miscreants never surrender to Kṛṣṇa. Therefore their mental speculations to arrive at the Absolute Truth bring them to the false conclusion that the ordinary living entity and Kṛṣṇa are one and the same. With such a false conviction, they think that the body of any human being is now simply covered by material nature and that as soon as one is liberated from this material body there is no difference between God and himself.
(Bhagavad-Gita-----9:12-----purport).