Material nature itself is constituted by three qualities: the mode of goodness, the mode of passion and the mode of ignorance. Above these modes there is eternal time, and by a combination of these modes of nature and under the control and purview of eternal time there are activities, which are called karma. These activities are being carried out from time immemorial, and we are suffering or enjoying the fruits of our activities. For instance, suppose I am a businessman and have worked very hard with intelligence and have amassed a great bank balance. Then I am an enjoyer. But then say I have lost all my money in business; then I am a sufferer. Similarly, in every field of life we enjoy the results of our work, or we suffer the results. This is called karma.
(Bhagavad-Gita---introduction).

Īśvara (the Supreme Lord), jīva (the living entity), prakṛti (nature), kāla (eternal time) and karma (activity) are all explained in the Bhagavad-gītā. Out of these five, the Lord, the living entities, material nature and time are eternal. The manifestation of prakṛti may be temporary, but it is not false. Some philosophers say that the manifestation of material nature is false, but according to the philosophy of Bhagavad-gītā or according to the philosophy of the Vaiṣṇavas, this is not so. The manifestation of the world is not accepted as false; it is accepted as real, but temporary. It is likened unto a cloud which moves across the sky, or the coming of the rainy season, which nourishes grains. As soon as the rainy season is over and as soon as the cloud goes away, all the crops which were nourished by the rain dry up. Similarly, this material manifestation takes place at a certain interval, stays for a while and then disappears. Such are the workings of prakṛti. But this cycle is working eternally. Therefore prakṛti is eternal; it is not false. The Lord refers to this as "My prakṛti." This material nature is the separated energy of the Supreme Lord, and similarly the living entities are also the energy of the Supreme Lord, although they are not separated but eternally related. So the Lord, the living entity, material nature and time are all interrelated and are all eternal. However, the other item, karma, is not eternal. The effects of karma may be very old indeed. We are suffering or enjoying the results of our activities from time immemorial, but we can change the results of our karma, or our activity, and this change depends on the perfection of our knowledge. We are engaged in various activities. Undoubtedly we do not know what sort of activities we should adopt to gain relief from the actions and reactions of all these activities, but this is also explained in the Bhagavad-gītā.
(Bhagavad-Gita---introduction).

We shall find, therefore, in this Bhagavad-gītā that the complete whole is comprised of the supreme controller, the controlled living entities, the cosmic manifestation,eternal time and karma, or activities, and all of these are explained in this text. All of these taken completely form the complete whole, and the complete whole is called the Supreme Absolute Truth. The complete whole and the complete Absolute Truth are the complete Personality of Godhead, Śrī Kṛṣṇa. All manifestations are due to His different energies. He is the complete whole.
(Bhagavad-Gita---introduction).

Ignorance can be removed by gradual acceptance of the principles of Kṛṣṇa consciousness. Kṛṣṇa consciousness is awakened by different types of sacrifices to the demigods, sacrifice to Brahman, sacrifice in celibacy, in household life, in controlling the senses, in practicing mystic yoga, in penance, in forgoing material possessions, in studying the Vedas, and in partaking of the social institution called varṇāśrama-dharma. All of these are known as sacrifice, and all of them are based on regulated action. But within all these activities, the important factor is self-realization. One who seeks that objective is the real student of Bhagavad-gītā, but one who doubts the authority of Kṛṣṇa falls back. One is therefore advised to study Bhagavad-gītā, or any other scripture, under a bona fide spiritual master, with service and surrender. A bona fide spiritual master is in the disciplic succession from time eternal, and he does not deviate at all from the instructions of the Supreme Lord as they were imparted millions of years ago to the sun-god, from whom the instructions of Bhagavad-gītā have come down to the earthly kingdom. One should, therefore, follow the path of Bhagavad-gītā as it is expressed in the Gītā itself and beware of self-interested people after personal aggrandizement who deviate others from the actual path. The Lord is definitely the supreme person, and His activities are transcendental. One who understands this is a liberated person from the very beginning of his study of Bhagavad-gītā.
(Bhagavad-Gita---4:42----purport).

Another feature of Bhagavad-gītā is that the actual truth is the Supreme Personality of Godhead, Kṛṣṇa. The Absolute Truth is realized in three features—impersonal Brahman, localized Paramātmā, and ultimately the Supreme Personality of Godhead, Kṛṣṇa. Perfect knowledge of the Absolute Truth means perfect knowledge of Kṛṣṇa. If one understands Kṛṣṇa, then all the departments of knowledge are part and parcel of that understanding. Kṛṣṇa is transcendental, for He is always situated in His eternal internal potency. The living entities are manifested of His energy and are divided into two classes, eternally conditioned and eternally liberated. Such living entities are innumerable, and they are considered fundamental parts of Kṛṣṇa. Material energy is manifested into twenty-four divisions. The creation is effected by eternal time, and it is created and dissolved by external energy. This manifestation of the cosmic world repeatedly becomes visible and invisible.
(Bhagavad-Gita---18:78----purport).

In Bhagavad-gītā five principal subject matters have been discussed: the Supreme Personality of Godhead, material nature, the living entities, eternal time and all kinds of activities. All is dependent on the Supreme Personality of Godhead, Kṛṣṇa. All conceptions of the Absolute Truth—impersonal Brahman, localized Paramātmā and any other transcendental conception—exist within the category of understanding the Supreme Personality of Godhead. Although superficially the Supreme Personality of Godhead, the living entity, material nature and time appear to be different, nothing is different from the Supreme. But the Supreme is always different from everything. Lord Caitanya's philosophy is that of "inconceivable oneness and difference." This system of philosophy constitutes perfect knowledge of the Absolute Truth.
(Bhagavad-Gita---18:78----purport).

Another name of the Paramātmā feature of the Lord is kāla, or eternal time. Eternal time is the witness of all our actions, good and bad, and thus resultant reactions are destined by Him. It is no use saying that we do not know why and for what we are suffering. We may forget the misdeed for which we may suffer at this present moment, but we must remember that Paramātmā is our constant companion, and therefore He knows everything, past, present and future. And because the Paramātmā feature of Lord Kṛṣṇa destines all actions and reactions, He is the supreme controller also. Without His sanction not a blade of grass can move. The living beings are given as much freedom as they deserve, and misuse of that freedom is the cause of suffering. The devotees of the Lord do not misuse their freedom, and therefore they are the good sons of the Lord. Others, who misuse freedom, are put into miseries destined by the eternal kāla. The kāla offers the conditioned souls both happiness and miseries. It is all predestined by eternal time. As we have miseries uncalled-for, so we may have happiness also without being asked, for they are all predestined by kāla. No one is therefore either an enemy or friend of the Lord. Everyone is suffering and enjoying the result of his own destiny. This destiny is made by the living beings in course of social intercourse. Everyone here wants to lord it over the material nature, and thus everyone creates his own destiny under the supervision of the Supreme Lord. He is all-pervading and therefore He can see everyone's activities. And because the Lord has no beginning or end, He is known also as theeternal time, kāla.
(Srimad Bhagavatam----1:8:28----purport).

There is control by time all over the space within the universe, as there is control by time all over the planets. All the big gigantic planets, including the sun, are being controlled by the force of air, as the clouds are carried by the force of air. Similarly, the inevitable kāla, or time, controls even the action of the air and other elements. Everything, therefore, is controlled by the supreme kāla, a forceful representative of the Lord within the material world. Thus Yudhiṣṭhira should not be sorry for the inconceivable action of time. Everyone has to bear the actions and reactions of time as long as one is within the conditions of the material world. Yudhiṣṭhira should not think that he had committed sins in his previous birth and is suffering the consequence. Even the most pious has to suffer the condition of material nature. But a pious man is faithful to the Lord, for he is guided by the bona fide brāhmaṇa and Vaiṣṇava following the religious principles. These three guiding principles should be the aim of life. One should not be disturbed by the tricks of eternal time. Even the great controller of the universe, Brahmājī, is also under the control of that time; therefore, one should not grudge being thus controlled by time despite being a true follower of religious principles.
(Srimad Bhagavatam----1:9:14----purport).

Insurmountable, eternal time imperceptibly overcomes those who are too much attached to family affairs and are always engrossed in their thought.
"I am now happy; I have everything in order; my bank balance is quite enough; I can now give my children enough estate; I am now successful; the poor beggar sannyāsīs depend on God, but they come to beg from me; therefore I am more than the Supreme God." These are some of the thoughts which engross the insanely attached householder who is blind to the passing of eternal time. Our duration of life is measured, and no one is able to enhance it even by a second against the scheduled time ordained by the supreme will. Such valuable time, especially for the human being, should be cautiously spent because even a second passed away imperceptibly cannot be replaced, even in exchange for thousands of golden coins amassed by hard labor. Every second of human life is meant for making an ultimate solution to the problems of life, i.e. repetition of birth and death and revolving in the cycle of 8,400,000 different species of life. The material body, which is subject to birth and death, diseases and old age, is the cause of all sufferings of the living being, otherwise the living being is eternal; he is never born, nor does he ever die. Foolish persons forget this problem. They do not know at all how to solve the problems of life, but become engrossed in temporary family affairs not knowing that eternal time is passing away imperceptibly and that their measured duration of life is diminishing every second, without any solution to the big problem, namely repetition of birth and death, disease and old age. This is called illusion.
(Srimad Bhagavatam----1:13:17----translation and purport).

Death cannot be checked by anyone or from any source within this material world. Hiraṇyakaśipu wanted to be immortal and underwent a severe type of penance by which the whole universe trembled, and Brahmā himself approached him to dissuade Hiraṇyakaśipu from such a severe type of penance. Hiraṇyakaśipu asked Brahmā to award him the blessings of immortality, but Brahmā said that he himself was subject to death, even in the topmost planet, so how could he award him the benediction of immortality? So there is death even in the topmost planet of this universe, and what to speak of other planets, which are far, far inferior in quality to Brahmaloka, the residing planet of Brahmā. Wherever there is the influence of eternal time, there is this set of tribulations, namely birth, disease, old age and death, and all of them are invincible.
(Srimad Bhagavatam----1:13:19----purport).

In the Bhagavad-gītā, five important factors of knowledge have been delineated pertaining to (1) the Supreme Lord, (2) the living being, (3) nature, (4) time and space and (5) the process of activity. Out of these, the Supreme Lord and the living being are qualitatively one. The difference between the two has been analyzed as the difference between the whole and the part and parcel. Nature is inert matter displaying the interaction of three different modes, and eternal time and unlimited space are considered to be beyond the existence of the material nature. Activities of the living being are different varieties of aptitudes which can entrap or liberate the living being within and without material nature. All these subject matters are concisely discussed in the Bhagavad-gītā, and later the subject matters are elaborated in the Śrīmad-Bhāgavatam for further enlightenment. Out of the five subjects, the Supreme Lord, the living entity, nature, and time and space are eternal, but the living entity, nature and time are under the direction of the Supreme Lord, who is absolute and completely independent of any other control. The Supreme Lord is the supreme controller.
(Srimad Bhagavatam----1:15:27----purport).

A conditioned soul is enwrapped in his fruitive activities by the force of eternal time. But the Supreme Lord, when He incarnates on the earth, is not influenced by kāla, or the material conception of past, present and future. The activities of the Lord are eternal, and they are manifestations of His ātma-māyā, or internal potency. All pastimes or activities of the Lord are spiritual in nature, but to the laymen they appear to be on the same level with material activities. It so appeared that Arjuna and the Lord were engaged in the Battle of Kurukṣetra as the other party was also engaged, but factually the Lord was executing His mission of incarnation and association with His eternal friend Arjuna. Therefore such apparently material activities of Arjuna did not drive him away from his transcendental position, but on the contrary revived his consciousness of the songs of the Lord, as He sang them personally.
(Srimad Bhagavatam----1:15:30----purport).

This phenomenal world is impersonally the representation of Vāsudeva because the ingredients of its creation, their interaction and the enjoyer of the resultant action, the living being, are all produced by the external and internal energies of Lord Kṛṣṇa. This is confirmed in the Bhagavad-gītā (7.4-5). The ingredients, namely earth, water, fire, air and sky, as well as the conception of material identity, intelligence and the mind, are produced of the external energy of the Lord. The living entity who enjoys the interaction of the above gross and subtle ingredients, as set up by eternal time, is an offshoot of internal potency, with freedom to remain either in the material world or in the spiritual world.
(Srimad Bhagavatam----2:5:14----purport).

The Supreme Lord, who is the leader of all living entities, can award all the qualities of His personality unto His devotees, including immortality and spiritual bliss. In the material world there is always anxiety or fearfulness in the hearts of all living entities, but the Lord, being Himself the supreme fearless, also awards the same quality of fearlessness to His pure devotees. Mundane existence is itself a kind of fear because in all mundane bodies the effects of birth, death, old age and disease always keep a living being compact in fear. In the mundane world, there is always the influence of time, which changes things from one stage to another, and the living entity, originally being avikāra, or unchangeable, suffers a great deal on account of changes due to the influence of time. The changing effects of eternal time are conspicuously absent in the immortal kingdom of God, which should therefore be understood to have no influence of time and therefore no fear whatsoever.
(Srimad Bhagavatam----2:6:18----purport).