Vedic knowledge is called śabda-pramāṇa. Another name is śruti. Śruti means that this knowledge has to be received simply by aural reception. The Vedas instruct that in order to understand transcendental knowledge, we have to hear from the authority. Transcendental knowledge is knowledge from beyond this universe. Within this universe is material knowledge, and beyond this universe is transcendental knowledge. We cannot even go to the end of the universe, so how can we go to the spiritual world? Thus to acquire full knowledge is impossible.
(Sri Ishopanishad).

Simply you have to hear from the authoritative sources. But you can make some experiment. But that is not very important thing. The important thing is to hear from the authorized person. That is important. Therefore, Vedas are called śruti. Śruti means aural reception. You have to hear. Just like when you are sleeping, all your other senses are not active. But ear, if somebody is coming, your enemy, to hurt you, and your friend says, "Mr. such-and-such, wake up, wake up," so you can hear and you wake up and see that somebody is coming. So the ear is very important. Śrotriyaṁ brahma-niṣṭham (MU 1.2.12). Anyone who has heard perfectly from the disciplic succession of spiritual master, he is perfect.
(Lecture on Bhagavad-Gita----2:13).

So this Kṛṣṇa consciousness movement is to know everything perfectly from the supreme authority, Kṛṣṇa. This is the process. Tad vijñānārthaṁ sa gurum eva abhigacchet (MU 1.2.12). In order to understand subject matter which is beyond our perception, you have to approach such authority who can inform you. Exactly in the same way: to understand who is my father is beyond my perception, beyond my speculation, but if I accept the authoritative statement of my mother, this is perfect knowledge. So there are three kinds of processes to understand or to advance in knowledge. One is direct perception, pratyakṣa. And the other is authority, and the other is śruti. Śruti means by hearing from the Supreme. So our process is śruti. Śruti means we hear from the highest authority. That is our process, and that is very easy. Highest authority, if He is not in default... Ordinary persons, they are in default.
(Lecture on Bhagavad-Gita----2:14).

Actually this Bhagavad-gītā is described by Vyāsadeva. So Vyāsadeva recorded it, writing. Otherwise it was being received through hearing. Before this Kali-yuga Vyāsadeva, he kept all Vedic literature in writing. Before that, there was no book. The knowledge was received through the ear, aural reception, śruti. Therefore it is called śruti. Śruti means the knowledge which is received by hearing. And the memory was very sharp. In those days, five thousand years ago, any man... Not any man, but the intelligent class of men... They were called brāhmaṇas. They used to receive knowledge from guru by hearing. They could memorize everything, once heard. In the Kali-yuga the memory is being reduced. The duration of life is being reduced. Peoples' mercifulness is being reduced. This is the symptom of Kali-yuga. Bodily strength—reduced. Therefore Vyāsadeva preferred it that the Vedic knowledge should be kept recorded in writing.
(Lecture on Bhagavad-Gita----9:1).

Any devotional sentiment which is not supported by Vedas, śruti... Śruti means Veda, and smṛti means corollary to the Veda, or things which are written in corroboration with the Vedic ideas... That is called smṛti. Just like Bhagavad-gītā is smṛti. Bhagavad-gītā, the purpose of Bhagavad-gītā is the same as the Vedas, but it is not directly Veda; therefore it is called smṛti. Śruti-smṛti-purāṇādi (Brs. 1.2.101). The Purāṇas, there are eighteen Purāṇas. Purāṇādi means Rāmāyaṇa, Mahābhārata also. Śruti-smṛti-purāṇādi-pañcarātra-vidhiṁ vinā (Brs. 1.2.101). Pañcarātra-vidhi, given by Nārada Muni for worshiping the Deity. We are worshiping the Deity under the pañcarātra-vidhi. So Rūpa Gosvāmī says that "Any devotional service which has no reference with śruti, smṛti, purāṇa, pañcarātra, that is simply a disturbance. Simply disturbance, creating disturbance." We cannot manufacture. Sometimes we are questioned, "Can we do this? Can we do that?" Of course, it is good. But there is no need of manufacturing some idea. Whatever idea is already there, follow strictly.
(Lecture on Srimad Bhagavatam----1:2:18).

Direct perception, that is evidence. People with poor fund of knowledge, they want direct perception of everything. That is not possible. Direct perception of everything is not possible. Therefore aitihya. Aitihya means historical, historical, paramparā, hearing, traditional. And the next first-class evidence is śruti. Śruti means to hear from the authority. That is śruti. Just like the example we have several times cited here that the evidence "Who is my father?" that evidence is to hear from my mother. That's all. There is no other evidence. The mother says that "This is your father. He is your father." This is śruti, hearing from the mother, authority. And we have no other authority to understand father. Similarly, we have to understand our supreme father from the śruti mother, Vedas mother, mother Vedic mother. We have to accept Vedas as mother, śruti. The Vedas are considered as mother, and the Purāṇas are considered as sister. That is explained. So, śruti-pramāṇa, śruti-pramāṇa.
(Lecture on Sri Caitanya Caritamrta----2:20:318-329).

One great sage, a learned sage, after finishing all kinds of Vedic literature... There are two kinds of Vedic literature. One is called śruti, and the other is called smṛti. Śruti means the original Vedic injunction which is coming through disciplic succession beginning from Kṛṣṇa down to this day. There are certain axiomatic truths which is called Vedic injunction. The axiomatic truth, as I have given you several times example that cow dung is pure... Now, your reason is, "Oh, you say the Vedic injunction that if you touch stool of any animal you have to take your bath, purify yourself. And the Vedic injunction says cow dung is pure. Oh, this is contradictory." No. Not contradictory. This is injunction. People are actually accepting this, no argument, and they are benefited by it. So axiomatic truth. How it is truth? You may not have sufficient intelligence, but if you go deep into the matter you will find it is all truth: "Yes, it is all right."
(Lecture on Sri Caitanya Caritamrta----2:22:5).

Smṛti means authorized books written in accordance to the Śruti. Śruti smṛti. Just like Bhagavad-gītā. Bhagavad-gītā is not directly Vedas, but the all the stanzas of Bhagavad-gītā, they're strictly according to the Nyāya-prasthāna and Śruti-prasthāna. So any book which is strictly written according to the Vedas and Vedāntas, that is also accepted as Vedic literature.
(Lecture on Bhagavad-Gita----1:40).

Smṛti means literature which is written according to Vedic principles. That is called smṛti.
(Lecture on Bhagavad-Gita----4:3).

Smṛti means statement derived from śruti. Just like Bhagavad-gītā is called smṛti, the Purāṇas are called smṛti.
(Lecture on Srimad Bhagavatam----1:8:40).

Śruti means Vedas, śāstras. Śruti-smṛti. And smṛti means books which follow the Vedic principles. Vedas... Suppose you write one book, or anything. If it is just according to the Vedic conclusion, then it is also... It is called smṛti. By remembering the Vedic conclusion... You cannot go beyond the Vedic conclusion. Then it is useless writing. Vedic conclusion must be there. The guide must be there. On that conclusion, if you write something, that is right, and if you deviate from that conclusion, then it is wrong. So we want to read authorized, right books. Not by imagination. You can write so many nonsense things by imagination. That is useless. You must remember what is the Vedic conclusion. So śruti-smṛti-pañcarātra-vidhim.
(Lecture on Srimad Bhagavatam----2:3:24).

The smṛti, the scriptures following the principles of Vedic knowledge, are considered the evidence of Vedic principles. There are twenty different types of scripture for following religious principles, and among them the scriptures of Manu and Yājñavalkya are considered to be all-pervading authorities. In the Yājñavalkya-smṛti it is said:
śruti-smṛti-sadācāraḥ
svasya ca priyam ātmanaḥ
samyak saṅkalpajaḥ kāmo
dharma-mūlam idaṁ smṛtam
One should learn human behavior from śruti, the Vedas, and from smṛti, the scriptures following the Vedic principles.
(Srimad Bhagavatam----7:11:7----purport).

The seven opulences listed—fame, fortune, fine speech, memory, intelligence, steadfastness and patience—are considered feminine. If a person possesses all of them or some of them he becomes glorious. If a man is famous as a righteous man, that makes him glorious. Sanskrit is a perfect language and is therefore very glorious. If after studying one can remember a subject matter, he is gifted with a good memory, or smṛti. And the ability not only to read many books on different subject matters but to understand them and apply them when necessary is intelligence (medhā), another opulence. The ability to overcome unsteadiness is called firmness or steadfastness (dhṛti). And when one is fully qualified yet is humble and gentle, and when one is able to keep his balance both in sorrow and in the ecstasy of joy, he has the opulence called patience (kṣamā).
(Bhagavad-Gita----10:34----purport).

Without understanding the Supreme Personality of Godhead in this way, one only creates a disturbance. In this age of Kali, so many gurus have sprung up, and because they do not refer to the śruti-smṛti-purāṇādi-pañcarātrika-vidhi, they are creating a great disturbance in the world in regard to understanding the Absolute Truth. However, those who follow the pāñcarātrikī-vidhi under the guidance of a proper spiritual master can understand the Absolute Truth.
(Srimad Bhagavatam----8:12:10----purport).

As Brahman and Paramātmā realization are imperfect realizations of the Absolute Truth, so the means of realizing Brahman and Paramātmā, i.e., the paths of jñāna and yoga, are also imperfect means of realizing the Absolute Truth. Devotional service which is based on the foreground of full knowledge combined with detachment from material association, and which is fixed by dint of the aural reception of the Vedānta-śruti, is the only perfect method by which the seriously inquisitive student can realize the Absolute Truth. Devotional service is not, therefore, meant for the less intelligent class of transcendentalists.
(Sri Caitanya Caritamrta----1:7:102----purport).

You have to accept anything from the authorized source. So according to Vedic civilization, all knowledge is received from the Vedas, perfect authorized source. Sruti-pramana, evidence from the sruti, from the Vedas, that is perfect. Therefore, according to Vedic civilization, if you want to establish something you have to quote the section or the injunction from the Vedas, Then it is perfect. In learned circle you cannot say anything hodge-podge. That will not be accepted. If you support your statement from the evidence of the Vedas, then you are accepted as authority.
(Lecture on Bhagavad-Gita----16:9).

By culture of such knowledge the human society can attain perfection of life. This perfection of life culminates in the realization of the Supreme Being, Viṣṇu. The śruti therefore directs that those who are actually advanced in learning should aspire for the service of Lord Viṣṇu. Unfortunately persons who are enamored by the external beauty of viṣṇu-māyā do not understand that culmination of perfection or self-realization depends on Viṣṇu.
(Srimad Bhagavatam----1:5:22----purport).

A sincere devotee must, therefore, be prepared to hear the Vedic literature like the Upaniṣads, Vedānta and other literatures left by the previous authorities or Gosvāmīs, for the benefit of his progress. Without hearing such literatures, one cannot make actual progress. And without hearing and following the instructions, the show of devotional service becomes worthless and therefore a sort of disturbance in the path of devotional service. Unless, therefore, devotional service is established on the principles of śruti, smṛti, purāṇa or pañcarātra authorities, the make-show of devotional service should at once be rejected. An unauthorized devotee should never be recognized as a pure devotee. By assimilation of such messages from the Vedic literatures, one can see the all-pervading localized aspect of the Personality of Godhead within his own self constantly. This is called samādhi.
(Srimad Bhagavatam----1:2:12----purport).

There are two kinds of śruti, or scripture. One is spoken by the Lord, and the other is spoken about the Lord and His devotees. Bhagavad-gītā is the former and Śrīmad-Bhāgavatam the latter. One must hear these scriptures repeatedly from reliable sources in order to become fixed in serious devotional service. Through engagement in such devotional service, one becomes freed from the contamination of māyā. It is stated in the Śrīmad-Bhāgavatam that hearing about the Supreme Personality of Godhead cleanses the heart of all contamination caused by the influence of the three modes of material nature. By continuous, regular hearing, the effects of the contamination of lust and greed to enjoy or lord it over material nature diminish, and when lust and greed diminish, one then becomes situated in the mode of goodness. This is the stage of Brahman realization, or spiritual realization. In this way one becomes fixed on the transcendental platform. Remaining fixed on the transcendental platform is liberation from material entanglement.
(Srimad Bhagavatam----3:27:21----purport).

Vedic knowledge is called śruti, learning by aural reception. One should actually receive the Vedic message from authorities like Kṛṣṇa and His representatives. Here Kṛṣṇa distinguishes everything very nicely, and one should hear from this source. Simply to hear like the hogs is not sufficient; one must be able to understand from the authorities. It is not that one should simply speculate academically. One should submissively hear from Bhagavad-gītā that these living entities are always subordinate to the Supreme Personality of Godhead. Anyone who is able to understand this, according to the Supreme Personality of Godhead, Śrī Kṛṣṇa, knows the purpose of the Vedas; no one else knows the purpose of the Vedas.
(Bhagavad-Gita----15:19----purport).

Doubt, misapprehension, correct apprehension, memory and sleep, as determined by their different functions, are said to be the distinct characteristics of intelligence.
Doubt is one of the important functions of intelligence; blind acceptance of something does not give evidence of intelligence. Therefore the word saṁśaya is very important; in order to cultivate intelligence, one should be doubtful in the beginning. But doubting is not very favorable when information is received from the proper source. In Bhagavad-gītā the Lord says that doubting the words of the authority is the cause of destruction.
As described in the Patañjali yoga system, pramāṇa-viparyaya-vikalpa-nidra-smṛtyaḥ. By intelligence only one can understand things as they are. By intelligence only can one understand whether or not he is the body. The study to determine whether one's identity is spiritual or material begins in doubt. When one is able to analyze his actual position, the false identification with the body is detected. This is viparyāsa. When false identification is detected, then real identification can be understood. Real understanding is described here as niścayaḥ, or proved experimental knowledge. This experimental knowledge can be achieved when one has understood the false knowledge. By experimental or proved knowledge, one can understand that he is not the body but spirit soul.
Smṛti means "memory," and svāpa means "sleep." Sleep is also necessary to keep the intelligence in working order. If there is no sleep, the brain cannot work nicely. In Bhagavad-gītā it is especially mentioned that persons who regulate eating, sleeping and other necessities of the body in the proper proportion become very successful in the yoga process. These are some of the aspects of the analytical study of intelligence as described in both the Patañjali yoga system and the Sāṅkhya philosophy system of Kapiladeva in Śrīmad-Bhāgavatam.
(Srimad Bhagavatam----3:26:30----translation and purport).