Text 1-2

arjuna uvaca
prakritim purusham caiva
kshetram kshetra-jnam eva ca
etad veditum icchami
jnanam jneyam ca keshava

sri-bhagavan uvaca
idam sariram kaunteya
kshetram ity abhidhiyate
etad yo vetti tam prahuh
kshetra-jna iti tad-vidah

Translation

Arjuna said: O my dear Krishna, I wish to know about prakriti [nature], purusha [the enjoyer], and the field and the knower of the field, and of knowledge and the object of knowledge.

The Supreme Personality of Godhead said: This body, O son of Kunti, is called the field, and one who knows this body is called the knower of the field. 

Commentary by Srila Prabhupada 

Arjuna was inquisitive about prakriti (nature), purusha (the enjoyer), kshetra (the field), kshetra-jna (its knower), and knowledge and the object of knowledge. When he inquired about all these, Krishna said that this body is called the field and that one who knows this body is called the knower of the field. This body is the field of activity for the conditioned soul. The conditioned soul is entrapped in material existence, and he attempts to lord it over material nature. And so, according to his capacity to dominate material nature, he gets a field of activity. That field of activity is the body. And what is the body? The body is made of senses. The conditioned soul wants to enjoy sense gratification, and, according to his capacity to enjoy sense gratification, he is offered a body, or field of activity. Therefore the body is called kshetra, or the field of activity for the conditioned soul. Now, the person, who should not identify himself with the body, is called kshetra-jna, the knower of the field. It is not very difficult to understand the difference between the field and its knower, the body and the knower of the body. Any person can consider that from childhood to old age he undergoes so many changes of body and yet is still one person, remaining. Thus there is a difference between the knower of the field of activities and the actual field of activities. A living conditioned soul can thus understand that he is different from the body. It is described in the beginning—dehino ’smin—that the living entity is within the body and that the body is changing from childhood to boyhood and from boyhood to youth and from youth to old age, and the person who owns the body knows that the body is changing. The owner is distinctly kshetra-jna. Sometimes we think, “I am happy,” “I am a man,” “I am a woman,” “I am a dog,” “I am a cat.” These are the bodily designations of the knower. But the knower is different from the body. Although we may use many articles—our clothes, etc.—we know that we are different from the things used. Similarly, we also understand by a little contemplation that we are different from the body. I or you or anyone else who owns the body is called kshetra-jna, the knower of the field of activities, and the body is called kshetra, the field of activities itself. 

In the first six chapters of Bhagavad-gita the knower of the body (the living entity) and the position by which he can understand the Supreme Lord are described. In the middle six chapters of the Bhagavad-gita the Supreme Personality of Godhead and the relationship between the individual soul and the Supersoul in regard to devotional service are described. The superior position of the Supreme Personality of Godhead and the subordinate position of the individual soul are definitely defined in these chapters. The living entities are subordinate under all circumstances, but in their forgetfulness they are suffering. When enlightened by pious activities, they approach the Supreme Lord in different capacities—as the distressed, those in want of money, the inquisitive, and those in search of knowledge. That is also described. Now, starting with the Thirteenth Chapter, how the living entity comes into contact with material nature and how he is delivered by the Supreme Lord through the different methods of fruitive activities, cultivation of knowledge, and the discharge of devotional service are explained. Although the living entity is completely different from the material body, he somehow becomes related. This also is explained.

Commentary by Sri Visvanatha Cakravarti Thakur

Text 1: Let me offer my respects to the Lord's bhakti, which, by its mercy, resides within jnana and other processes to a small degree in order to bring about success in those processes. In the last six chapters of the Gita, jnana mixed with bhakti is delineated. Within these six chapters, kevala bhakti also is shown as supreme, indirectly. In the thirteenth chapter, the body where jiva and paramatma reside, the practice of jnana, the jiva and prakrti are described.

In the middle six chapters, it was mentioned that by kevala bhakti one attains the Lord, bhagavan, and three other methods of worship, starting with those who worship themselves, were then described. Liberation arising from jnana mixed with bhakti practiced by followers of niskama karma yoga and the process of jnana were briefly described in the first six chapters. The third six chapters commence to explain this jnana in detail by first examining the field, the knower of the field, the process of knowledge and the object of knowledge.

Text 2: This verse answers the question of what is the field and who is the knower of the field. This body with senses, the abode of enjoyment, is the field, since it is the basis for sprouting the tree of repeated births. He who knows that body in terms of "I and mine" in the conditioned state due his attachment to that body, and who, being devoid of the conception of "I and mine" in the liberated state, knows that he is not attached to that body, is the jiva, He is called the knower of the field, situated in these two conditions. Like one who ploughs the field, he is the knower of the field, and is the enjoyer of the fruit. The Lord says:

adanti caikam phalam asya grdhra
grame-cara ekam aranya-vasah
hamsa ya ekam bahu-rupam ijyair
maya-mayam veda sa veda vedam

Those lusty after material enjoyment and dedicated to family life enjoy one of the tree's fruits, and swanlike men in the renounced order of life enjoy the other fruit One who with the help of the bona fide spiritual masters can understand this tree to be a manifestation of the potency of the one Supreme Truth appearing in many forms actually knows the meaning of the Vedic literature. SB 11.12.23

The meaning of this verse from Bhagavatam is as follows. Grdhrah means "those who desire." Those who have desire, who move in the village, the conditioned jivas, eat one fruit of the tree called distress, because the tree has the ability to generate distress, even in ripening into svarga. The swans, which live in the forest, the liberated jivas, eat another fruit called happiness, because the tree has ability to generate happiness in the form of liberation. Thus, the one tree of samsara has many forms since it has the capacity to let one attain hell, svarga, and liberation. It is called maya mayam, made of maya, because it is generated from the Lord's maya sakti. He who knows this tree with the help of the worshipable gurus (ijyaih) knows the Vedas.

Tad vidah in the Gita verse refers to those who know the field and the knower of the field.