In the language of Bhagavad-gītā, the spirit soul is called kṣetrajña, the knower or tiller of the field, whereas the body and mind, the coverings of the spirit soul, are called kṣetra, or the field. In the eleventh chapter of Bhagavad-gītā, the Personality of Godhead, Śrī Kṛṣṇa, discusses the subject matter of kṣetra, kṣetrajña, and also prakṛti (nature, or the phenomenal world, which is enjoyed) and puruṣa (the enjoyer of the phenomenal world). Lord Kṛṣṇa explains that all actions and reactions that take place in this phenomenal world are the actions and reactions of this combination of kṣetra and kṣetrajña, or nature and the enjoyer of nature. For instance, rice paddy is produced by the action and reaction of the field and the tiller, or a child is begotten by the combination of prakṛti, the enjoyed, and puruṣa, the enjoyer. In the same way, whatever we see in the phenomenal world is produced by this combination of kṣetra and kṣetrajña.
This kṣetrajña is the living spirit, whereas the kṣetra is the material which is lorded over. Physics, chemistry, astronomy, pharmacology, economics, sexology, and other material sciences deal with the materials of kṣetra. But the science that deals with spiritual existence—pertaining to kṣetrajña—is called transcendental knowledge. Real culture of knowledge, therefore, pertains not to kṣetra but to kṣetrajña. We shall get full opportunity to discuss all these subjects more elaborately, but for the present we may be satisfied simply by knowing that the kṣetrajña (puruṣa, or enjoyer) is the central objective of all knowledge, because it is this kṣetrajña alone that creates everything in conjunction with the material body and mind and the allied physical elements.
The kṣetrajña is the eternal spirit, whereas the kṣetra is matter, which is temporary and ephemeral. This eternal truth is summarized in the Vedas in the aphorism brahma satyaṁ jagan mithyā: "Spirit is fact and the world is a false shadow." By "false shadow" one should understand that the world is temporary, existing only for the time being. But one should not make the mistake of thinking the world has no existence at all. I really possess my temporary material body and mind, and I must not make myself a laughing stock by denying the existence of my body and mind. At the same time, I must always remember that the body and mind are temporary arrangements. However, the spirit encaged by this body and mind is eternal truth and indestructible. No one can destroy the eternal spirit—that is what we need to understand at the present moment. The indestructible spirit is thus above the conception of violence and nonviolence.
Today, the whole world is mad after the culture of knowledge in relation to temporary arrangements for the gross material body and the subtle material mind. But more important than the body and mind is the spirit, which has been set aside without any proper culture of knowledge. As a result, the darkness of nescience has overshadowed the world and has brought about great unrest, disturbance, and distress. How long can one enjoy external happiness? It is like soaping the outer garments without putting any nourishment into the stomach.
But this eternal truth, the indestructible spirit, does exist as the living entity in each and every body. He is very minute and is finer than the finest atom. Learned experts have attempted to make a measurement of this living spirit. They say that the living spirit, the soul proper, can be measured approximately as one ten-thousandth part of the tip of a hair.