One who works for the satisfaction of the Lord only, however difficult such work may be, is working under the principles of buddhi-yoga and finds himself always in transcendental bliss. By such transcendental engagement, one achieves all transcendental understanding automatically, by the grace of the Lord, and thus his liberation is complete in itself, without his making extraneous endeavors to acquire knowledge. There is much difference between work in Kṛṣṇa consciousness and work for fruitive results, especially in the matter of sense gratification for achieving results in terms of family or material happiness. Buddhi-yoga is therefore the transcendental quality of the work that we perform.
(Bhagavad-Gita------2:39------purport).

In the Vedas the symptoms of the lower animals are described as follows: athetareṣāṁ paśūnāḥ aśanāpipāse evābhivijñānaṁ na vijñātaṁ vadanti na vijñātaṁ paśyanti na viduḥ śvastanaṁ na lokālokāv iti; yad vā, bhūri-tamaso bahu-ruṣaḥ ghrāṇenaiva jānanti hṛdyaṁ prati svapriyaṁ vastv eva vindanti bhojana-śayanādy-arthaṁ gṛhṇanti. "Lower animals have knowledge only of their hunger and thirst. They have no acquired knowledge, no vision. Their behavior exhibits no dependence on formalities. Extensively ignorant, they can know their desirables only by smell, and by such intelligence only can they understand what is favorable and unfavorable. Their knowledge is concerned only with eating and sleeping." Therefore, even the most ferocious lower animals, such as tigers, can be tamed simply by regularly supplying meals and accommodations for sleeping. Only snakes cannot be tamed by such an arrangement.
(Srimad Bhagavatam-------3:10:21------purport).

The living entities, as separate parts and parcels of the Supreme, have a purpose to fulfill. Having forgotten that purpose since time immemorial, they are situated in different bodies, as men, animals, demigods, etc. Such bodily differences arise from forgetfulness of the transcendental service of the Lord. But when one is engaged in transcendental service through Kṛṣṇa consciousness, one becomes at once liberated from this illusion. One can acquire such pure knowledge only from the bona fide spiritual master and thereby avoid the delusion that the living entity is equal to Kṛṣṇa. Perfect knowledge is that the Supreme Soul, Kṛṣṇa, is the supreme shelter for all living entities, and giving up such shelter, the living entities are deluded by the material energy, imagining themselves to have a separate identity. Thus, under different standards of material identity, they become forgetful of Kṛṣṇa. When, however, such deluded living entities become situated in Kṛṣṇa consciousness, it is to be understood that they are on the path of liberation, as confirmed in the Bhāgavatam (2.10.6): muktir hitvānyathā-rūpaṁ svarūpeṇa vyavasthitiḥ. Liberation means to be situated in one's constitutional position as an eternal servitor of Kṛṣṇa (Kṛṣṇa consciousness).
(Bhagavad-Gita------4:35------purport).

Kṛṣṇa, of course, means Kṛṣṇa and His confidential devotees; Kṛṣṇa is not alone. When we speak of Kṛṣṇa, "Kṛṣṇa" means Kṛṣṇa in His name, in His form, in His qualities, in His abode and in His associates. Kṛṣṇa is never alone, for the devotees of Kṛṣṇa are not impersonalists. For example, a king is always associated with his secretary, his commander, his servant and so much paraphernalia. As soon as we accept Kṛṣṇa and His associates as our preceptors, no ill effects can destroy our knowledge. In the material world theknowledge which we acquire may change because of the influence of time, but nevertheless the conclusions received from Bhagavad-gītā, directly from the speeches of the Supreme Lord, Kṛṣṇa, can never change. There is no use interpreting Bhagavad-gītā; it is eternal.
(Srimad Bhagavatam-------3:25:38------purport).

After acquiring perfect transcendental knowledge, one acquires qualitative equality with the Supreme Personality of Godhead, becoming free from the repetition of birth and death. One does not, however, lose his identity as an individual soul. It is understood from Vedic literature that the liberated souls who have reached the transcendental planets of the spiritual sky always look to the lotus feet of the Supreme Lord, being engaged in His transcendental loving service. So, even after liberation, the devotees do not lose their individual identities.
(Bhagavad-Gita------14:2------purport).

The primordial matter, or prakṛti, material nature, consisting of three modes, generates four groups of five. The first group is called elementary and consists of earth, water, fire, air and ether. The second group of five is called tan-mātra, referring to the subtle elements (sense objects): sound, touch, form, taste and smell. The third group is the five sense organs for acquiring knowledge: eyes, ears, nose, tongue and skin. The fourth group is the five working senses: speech, hands, feet, anus and genitals. Some say that there are five groups of five. One group is the sense objects, one is the five elements, one is the five sense organs for acquiring knowledge, another is the senses for working, and the fifth group is the five deities who control these divisions.
(Srimad Bhagavatam-------3:20:13------purport).

Prescribed duties of the human being, as directed in the Vedas, can gradually purify the sinful mind of the conditioned soul and raise him to the stage of knowledge. The purified stage of acquiring knowledge becomes the basis of devotional service to the Lord. As long as one is engaged in researching the solution of the problems of life, his knowledgeis called jñāna, or purified knowledge, but on realizing the actual solution of life, one becomes situated in the devotional service of the Lord.
(Srimad Bhagavatam-------1:15:27------purport).

The living condition in material existence depends more or less on one's intelligence and powerful living energy. Intelligence to counteract the hard struggle for existence is assisted by the senses for acquiring knowledge, and the living energy maintains himself by manipulating the active organs, like the hands and legs. But on the whole, the struggle for existence is an exertion of the mode of passion. Therefore all the sense organs, headed by intelligence and the living energy, prāṇa, are different products and by-products of the second mode of nature, called passion.
(Srimad Bhagavatam-------2:5:31------purport).

Devotional service to the Lord is the ultimate issue, and therefore those who are directly engaged in the devotional service of the Lord not only acquire the necessaryknowledge in spiritual science, but also attain detachment from material connection and are thus promoted to the kingdom of God by complete liberation, as stated in the Bhagavad-gītā (14.26):
māṁ ca yo 'vyabhicāreṇa
bhakti-yogena sevate
sa guṇān samatītyaitān
brahma-bhūyāya kalpate
Even in the nonliberated stage, a living entity can be directly engaged in the transcendental loving service of the Personality of Godhead Lord Kṛṣṇa or His plenary expansions like Rāma and Narasiṁha.
(Srimad Bhagavatam-------2:7:3------purport).

There are different levels of acquired knowledge—direct knowledge, knowledge received from authorities, transcendental knowledge, knowledge beyond the senses, and finally spiritual knowledge. When one surpasses the stage of acquiring knowledge by the descending process, he is immediately situated on the transcendental platform. Dhruva Mahārāja, being liberated from the material concept of life, was situated in transcendental knowledge and could perceive the presence of a transcendental airplane which was as brilliant as the full moonlight. This is not possible in the stages of direct or indirect perception of knowledge. Such knowledge is a special favor of the Supreme Personality of Godhead. One can, however, rise to this platform of knowledge by the gradual process of advancing in devotional service, or Kṛṣṇa consciousness.
(Srimad Bhagavatam-------4:12:19------purport).

As explained in the Second Chapter of the First Canto:
vāsudeve bhagavati
bhakti-yogaḥ prayojitaḥ
janayaty āśu vairāgyaṁ
jñānaṁ ca yad ahaitukam
"By rendering devotional service unto the Personality of Godhead, Śrī Kṛṣṇa, one immediately acquires causeless knowledge and detachment from the world." (S.B.1:2:7) Thus one must engage in the service of Vāsudeva without material desire, mental speculation or fruitive activity.
(Srimad Bhagavatam-------4:29:36:37------purport).

Worship of the Lord is not at all expensive. One can offer the Lord a leaf, a flower, a little fruit and some water. The Supreme Lord accepts these offerings when they are offered with love and devotion. In this way, one can become freed from material desires. As long as one maintains material desires, he cannot be happy. As soon as one engages in the devotional service of the Lord, his mind is purified of all material desires. Then one becomes fully satisfied.
sa vai puṁsāṁ paro dharmo
yato bhaktir adhokṣaje
ahaituky apratihatā
yayātmā suprasīdati
vāsudeve bhagavati
bhakti-yogaḥ prayojitaḥ
janayaty āśu vairāgyaṁ
jñānaṁ ca yad ahaitukam
"The supreme occupation (dharma) for all humanity is that by which men can attain to loving devotional service unto the transcendent Lord. Such devotional service must be unmotivated and uninterrupted in order to completely satisfy the self. By rendering devotional service unto the Personality of Godhead, Śrī Kṛṣṇa, one immediately acquires causeless knowledge and detachment from the world." (SB 1.2.6-7)
(Srimad Bhagavatam-------5:7:11------purport).