Any layman can understand that the material body minus consciousness is a dead body, and this consciousness cannot be revived in the body by any means of material administration. Therefore, consciousness is not due to any amount of material combination, but to the spirit soul. In the Muṇḍaka Upaniṣad (3.1.9) the measurement of the atomic spirit soul is further explained:
eṣo 'ṇur ātmā cetasā veditavyo
yasmin prāṇaḥ pañcadhā saṁviveśa
prāṇaiś cittaṁ sarvam otaṁ prajānāṁ
yasmin viśuddhe vibhavaty eṣa ātmā
"The soul is atomic in size and can be perceived by perfect intelligence. This atomic soul is floating in the five kinds of air (prāṇa, apāna, vyāna, samāna and udāna), is situated within the heart, and spreads its influence all over the body of the embodied living entities. When the soul is purified from the contamination of the five kinds of material air, its spiritual influence is exhibited."
(Bhagavad-Gita-----2:17-----purport).

There are various theories regarding consciousness. Here in Bhagavad-gītā the example of the sun and the sunshine is given. As the sun is situated in one place but is illuminating the whole universe, so a small particle of spirit soul, although situated in the heart of this body, is illuminating the whole body by consciousness. Thus consciousness is the proof of the presence of the soul, as sunshine or light is the proof of the presence of the sun. When the soul is present in the body, there is consciousness all over the body, and as soon as the soul has passed from the body there is no more consciousness. This can be easily understood by any intelligent man. Therefore consciousness is not a product of the combinations of matter. It is the symptom of the living entity. The consciousness of the living entity, although qualitatively one with the supreme consciousness, is not supreme, because the consciousness of one particular body does not share that of another body. But the Supersoul, which is situated in all bodies as the friend of the individual soul, is conscious of all bodies. That is the difference between supreme consciousness and individual consciousness.
(Bhagavad-Gita-----13:34-----purport).

The soul is full of knowledge, or full always with consciousness. Therefore, consciousness is the symptom of the soul. Even if one does not find the soul within the heart, where he is situated, one can still understand the presence of the soul simply by the presence of consciousness. Sometimes we do not find the sun in the sky owing to clouds, or for some other reason, but the light of the sun is always there, and we are convinced that it is therefore daytime. As soon as there is a little light in the sky early in the morning, we can understand that the sun is in the sky. Similarly, since there is some consciousness in all bodies—whether man or animal—we can understand the presence of the soul. This consciousness of the soul is, however, different from the consciousness of the Supreme because the supreme consciousness is all-knowledge—past, present and future. The consciousness of the individual soul is prone to be forgetful. When he is forgetful of his real nature, he obtains education and enlightenment from the superior lessons of Kṛṣṇa.
(Bhagavad-Gita-----2:20-----purport).

Consciousness is the sign of the living entity, or the soul. The existence of the soul is manifest in the form of consciousness, called jñāna-śakti. The total consciousness is that of the gigantic virāṭ-rūpa, and the same consciousness is exhibited in individual persons. The activity of consciousness is performed through the air of life, which is of ten divisions. The airs of life are called prāṇa, apāna, udāna, vyāna and samāna and are also differently qualified as nāga, kūrma, kṛkara, devadatta and dhanañjaya. The consciousness of the soul becomes polluted by the material atmosphere, and thus various activities are exhibited in the false ego of bodily identification. These various activities are described in Bhagavad-gītā (2.41) as bahu-śākhā hy anantāś ca buddhayo 'vyavasāyinām. The conditioned soul is bewildered into various activities for want of pure consciousness. In pure consciousness the activity is one. The consciousness of the individual soul becomes one with the supreme consciousness when there is complete synthesis between the two.
(Srimad Bhagavatam-----3:6:7------purport).

The monist believes that there is only one consciousness, whereas the sātvatas, or the devotees, believe that although there is undoubtedly one consciousness, they are one because there is agreement. The individual consciousness is advised to dovetail with the supreme consciousness, as instructed by the Lord in Bhagavad-gītā (18.66): sarva-dharmān parityajya mām ekaṁ śaraṇaṁ vraja. The individual consciousness (Arjuna) is advised to dovetail with the supreme consciousness and thus maintain his conscious purity. It is foolish to try to stop the activities of consciousness, but they can be purified when they are dovetailed with the Supreme. This consciousness is divided into three modes of self-identification according to the proportion of purity: ādhyātmika, or self-identification with the body and mind, ādhibhautika, or self-identification with the material products, and ādhidaivika, or self-identification as a servant of the Lord. Of the three, ādhidaivika self-identification is the beginning of purity of consciousness in pursuance of the desire of the Lord.
(Srimad Bhagavatam-----3:6:7------purport).

According to the Vedānta-sūtras (śāstra-yonitvāt), the Supreme Lord is the author of all revealed scriptures, and all revealed scriptures are for knowing the Supreme Lord. Veda means knowledge that leads to the Lord. The Vedas are made just to revive the forgotten consciousness of the conditioned souls, and any literature not meant for reviving God consciousness is rejected at once by the nārāyaṇa-para devotees. Such deluding books of knowledge, not having Nārāyaṇa as their aim, are not at all knowledge, but are the playgrounds for crows who are interested in the rejected refuse of the world. Any book of knowledge (science or art) must lead to the knowledge of Nārāyaṇa; otherwise it must be rejected. That is the way of advancement of knowledge.
(Srimad Bhagavatam-----2:5:15------purport).

All descriptions of the child's bodily situation in the womb of the mother are beyond our conception. It is very difficult to remain in such a position, but still the child has to remain. Because his consciousness is not very developed, the child can tolerate it, otherwise he would die. That is the benediction of māyā, who endows the suffering body with the qualifications for tolerating such terrible tortures.
(Bhagavad-Gita-----3:31:7-----purport).

The word anusasmāra is very significant. God consciousness is not imaginary or concocted. The devotee who is pure and advanced realizes God as He is, Mahārāja Pratīha did so, and due to his direct realization of Lord Viṣṇu, he propagated self-realization and became a preacher. A real preacher cannot be bogus; he must first of all realize Lord Viṣṇu as He is.
(Bhagavad-Gita-----5:15:4-----purport).

Similarly, change of body means I forget this body. Last life, I had some body, but I cannot say what kind of body I had. Of course, there is science, astronomy and other things. They can speak, but I don't believe it. Or even if I believe, what can I do with that? So we are changing. But the consciousness is not vināśi. Avināśi tu tad viddhi yena sarvam idaṁ tatam. So because the consciousness is not changed, it is creating another body according to consciousness.
(Lecture on Bhagavad-Gita-----13:20).

So what is spread all over the body? Consciousness. That is spread all over the body. Everyone can understand that if I pinch any part of my body or your any part of the body, you feel pain. Or similarly, if you get some other facilities, pleasure, so pains and pleasures are felt so long there is consciousness. Any man can understand. And as soon as the consciousness is not there, sometimes we are made unconsciousness by drugs, by chloroform and other anaesthetic medicine, or by nature, unconscious stage.
(Lecture on Bhagavad-Gita-----2:17).

So this instruction was first given to Arjuna, that "Arjuna! You are arguing with Me just like a very learned scholar, but you are fool number one, and you just try to understand this first of all, that that thing which is spread all over..." Avināśi tu tad viddhi yena sarvam idaṁ tatam. Yena sarvam idam. Sarvam means the whole body, idam. But your consciousness is not spread in others' bodies. You must know it. If you say that "My consciousness is spread all over the universe," that is also another mistake. Your consciousness is limited within your body.
(Lecture on Bhagavad-Gita-----4:1).

Now suppose we are also conscious. So what is the nature of our consciousness? Our consciousness is that I know directly everything of my body, or of my self. But I do not know indirectly about yourself. I cannot say what is going on in your mind, in your body, what pains and pleasure you are feeling. But I can speak about myself that "I am feeling like this. I am thinking like this. I am willing like this." That I can say. So my consciousness is not perfect. It is perfect so far I am concerned. But I, my consciousness does not spread upon you. But here it is said, anvayāt itarataḥ abhijñaḥ artheṣu abhijñaḥ. "The Absolute Truth knows everything, directly and indirectly."
(Lecture on Srimad Bhagavatam-----1:2:25).

As spirit is not annihilated even after the annihilation of the material body, so also spiritual consciousness is not annihilated. Śrī Nārada developed this spiritual consciousness even when he had his material body in the previous kalpa. Consciousness of the material body means spiritual consciousness expressed through the medium of a material body. This consciousness is inferior, destructible and perverted. But superconsciousness of the supramind in the spiritual plane is as good as the spirit soul and is never annihilated.
(Srimad Bhagavatam-----1:6:4------purport).

The living entities are described as ananta, or unlimited; nonetheless, they are said to belong to 8,400,000 species. As stated in the Viṣṇu Purāṇa:
jala-jā nava-lakṣāṇi sthāvarā lakṣa-viṁśati
kṛmayo rudra-saṅkhyakāḥ pakṣiṇāṁ daśa-lakṣaṇam
triṁśal-lakṣāṇi paśavaḥ catur-lakṣāṇi mānuṣāḥ
"There are 900,000 species living in the water. There are also 2,000,000 nonmoving living entities (sthāvara), such as trees and plants. There are also 1,100,000 species of insects and reptiles, and there are 1,000,000 species of birds. As far as quadrupeds are concerned, there are 3,000,000 varieties, and there are 400,000 human species." Some of these species may exist on one planet and not on another, but in any case within all the planets of the universe—and even in the sun—there are living entities. This is the verdict of the Vedic literatures. As the Bhagavad-gītā (2.20) confirms:
na jāyate mriyate vā kadācin
nāyaṁ bhūtvā bhavitā vā na bhūyaḥ
ajo nityaḥ śāśvato ‘yaṁ purāṇo
na hanyate hanyamāne śarīre
"For the soul there is neither birth nor death at any time. He has not come into being, does not come into being, and will not come into being. He is unborn, eternal, ever-existing and primeval. He is not slain when the body is slain."
Since the living entities are never annihilated, they simply transmigrate from one life form to another. Thus there is an evolution of forms according to the degree of developed consciousness. One experiences different degrees of consciousness in different forms. A dog's consciousness is different from a man's. Even within a species we find that a father's consciousness is different from his son's and that a child's consciousness is different from a youth's. Just as we find different forms, we find different states of consciousness. When we see different states of consciousness, we may take it for granted that the bodies are different. In other words, different types of bodies depend on different states of consciousness. This is also confirmed in the Bhagavad-gītā (8.6):
yaṁ yaṁ vāpi smaran bhāvaṁ tyajanty ante kalevaram
taṁ tam evaiti kaunteya sadā tad bhāva-bhāvitaḥ
“One's consciousness at the time of death determines one's type of body in the next life.” This is the process of transmigration of the soul. A variety of bodies is already there; we change from one body to another in terms of our consciousness.
(Sri Caitanya Caritamrta-----2:19:138-----purport).

The purification of the conditioned soul necessitates purification of his consciousness. By the presence of consciousness, the presence of the transcendental soul is verified, and as soon as consciousness leaves the body, the material body is not active. Consciousness is perceived, therefore, by activities. The theory put forward by empiric philosophers that consciousness can remain in an inactive state is the proof of their poor fund of knowledge. One should not become unchaste by stopping the activities of pure consciousness. If the activities of pure consciousness are stopped, certainly the conscious living force will be otherwise engaged because unless engaged the consciousness has no standing. Consciousness cannot be silent, even for a moment. When the body does not act, the consciousness acts in the form of dreams. Unconsciousness is artificial; by induced extraneous help it remains for a limited period, but when the intoxication of the drug is finished or when one is awake, the consciousness again acts earnestly.
(Srimad Bhagavatam-----3:6:36------purport).

Every living entity is covered by a subtle and gross body. The subtle body is the covering of mind, ego, intelligence and consciousness. It is said in the scriptures that the constables of Yamarāja cover the subtle body of the culprit and take him to the abode of Yamarāja to be punished in a way that he is able to tolerate. He does not die from this punishment because if he died, then who would suffer the punishment? It is not the business of the constables of Yamarāja to put one to death. In fact, it is not possible to kill a living entity because factually he is eternal; he simply has to suffer the consequences of his activities of sense gratification.
(Srimad Bhagavatam-----3:30:20------purport).

The senses are different outlets for the activities of lust. Lust is reserved within the body, but it is given vent through the senses. Therefore, the senses are superior to the body as a whole. These outlets are not in use when there is superior consciousness, or Kṛṣṇa consciousness. In Kṛṣṇa consciousness the soul makes direct connection with the Supreme Personality of Godhead; therefore the hierarchy of bodily functions, as described here, ultimately ends in the Supreme Soul. Bodily action means the functions of the senses, and stopping the senses means stopping all bodily actions. But since the mind is active, then even though the body may be silent and at rest, the mind will act—as it does during dreaming. But above the mind is the determination of the intelligence, and above the intelligence is the soul proper. If, therefore, the soul is directly engaged with the Supreme, naturally all other subordinates, namely, the intelligence, mind and senses, will be automatically engaged. In the Kaṭha Upaniṣad there is a similar passage, in which it is said that the objects of sense gratification are superior to the senses, and mind is superior to the sense objects. If, therefore, the mind is directly engaged in the service of the Lord constantly, then there is no chance that the senses will become engaged in other ways. This mental attitude has already been explained. paraṁ dṛṣṭvā nivartate. If the mind is engaged in the transcendental service of the Lord, there is no chance of its being engaged in the lower propensities. In the Kaṭha Upaniṣad the soul has been described as mahān, the great. Therefore the soul is above all-namely, the sense objects, the senses, the mind and the intelligence. Therefore, directly understanding the constitutional position of the soul is the solution of the whole problem.
(Bhagavad-Gita-----3:42-----purport).

To become purified of material contamination is the necessary qualification for becoming one of the associates of the Lord. No one can become an associate of the Lord or can go back to Godhead without such purification. Mahārāja Yudhiṣṭhira, therefore, to become spiritually pure, at once gave up his royal opulence, relinquishing his royal dress and garments. The kaṣāya, or saffron loincloth of a sannyāsī, indicates freedom from all attractive material garments, and thus he changed his dress accordingly. He became disinterested in his kingdom and family and thus became free from all material contamination, or material designation. People are generally attached to various kinds of designations—the designations of family, society, country, occupation, wealth, position and many others. As long as one is attached to such designations, he is considered materially impure. The so-called leaders of men in the modern age are attached by national consciousness, but they do not know that such false consciousness is also another designation of the materially conditioned soul; one has to relinquish such designations before one can become eligible to go back to Godhead. Foolish people adore such men who die in national consciousness, but here is an example of Mahārāja Yudhiṣṭhira, a royal king who prepared himself to leave this world without such national consciousness. And yet he is remembered even today because he was a great pious king, almost on the same level with the Personality of Godhead Śrī Rāma. And because people of the world were dominated by such pious kings, they were happy in all respects, and it was quite possible for such great emperors to rule the world.
(Srimad Bhagavatam-----1:15:40------purport).