Lord Kṛṣṇa descends specifically to reestablish the real purpose of life when man forgets that purpose. Even then, out of many, many human beings who awaken, there may be one who actually enters the spirit of understanding his position, and for him this Bhagavad-gītā is spoken. Actually we are all swallowed by the tigress of nescience, but the Lord is very merciful upon living entities, especially human beings. To this end He spoke the Bhagavad-gītā, making His friend Arjuna His student.
Being an associate of Lord Kṛṣṇa, Arjuna was above all ignorance, but Arjuna was put into ignorance on the Battlefield of Kurukṣetra just to question Lord Kṛṣṇa about the problems of life so that the Lord could explain them for the benefit of future generations of human beings and chalk out the plan of life. Then man could act accordingly and perfect the mission of human life.
The subject of the Bhagavad-gītā entails the comprehension of five basic truths. First of all, the science of God is explained and then the constitutional position of the living entities, jīvas. There is īśvara, which means the controller, and there are jīvas, the living entities which are controlled. If a living entity says that he is not controlled but that he is free, then he is insane. The living being is controlled in every respect, at least in his conditioned life. So in the Bhagavad-gītā the subject matter deals with the īśvara, the supreme controller, and the jīvas, the controlled living entities. Prakṛti (material nature) and time (the duration of existence of the whole universe or the manifestation of material nature) and karma (activity) are also discussed. The cosmic manifestation is full of different activities. All living entities are engaged in different activities. From Bhagavad-gītā we must learn what God is, what the living entities are, what prakṛti is, what the cosmic manifestation is, how it is controlled by time, and what the activities of the living entities are.
Out of these five basic subject matters in Bhagavad-gītā it is established that the Supreme Godhead, or Kṛṣṇa, or Brahman, or the supreme controller, or Paramātmā—you may use whatever name you like—is the greatest of all. The living beings are in quality like the supreme controller. For instance, the Lord has control over the universal affairs of material nature, as will be explained in the later chapters of Bhagavad-gītā. Material nature is not independent. She is acting under the directions of the Supreme Lord. As Lord Kṛṣṇa says, mayādhyakṣeṇa prakṛtiḥ sūyate sa-carācaram: "This material nature is working under My direction." When we see wonderful things happening in the cosmic nature, we should know that behind this cosmic manifestation there is a controller. Nothing could be manifested without being controlled. It is childish not to consider the controller. For instance, a child may think that an automobile is quite wonderful to be able to run without a horse or other animal pulling it, but a sane man knows the nature of the automobile's engineering arrangement. He always knows that behind the machinery there is a man, a driver. Similarly, the Supreme Lord is the driver under whose direction everything is working. Now the jīvas, or the living entities, have been accepted by the Lord, as we will note in the later chapters, as His parts and parcels. A particle of gold is also gold, a drop of water from the ocean is also salty, and similarly we the living entities, being part and parcel of the supreme controller, īśvara, or Bhagavān, Lord Śrī Kṛṣṇa, have all the qualities of the Supreme Lord in minute quantity because we are minute īśvaras, subordinate īśvaras. We are trying to control nature, as presently we are trying to control space or planets, and this tendency to control is there because it is in Kṛṣṇa. But although we have a tendency to lord it over material nature, we should know that we are not the supreme controller. This is explained in Bhagavad-gītā.