sri bhagavaan uvaacha
    idam shareeram kaunteya kshetramityabhidheeyate
    etadyo vetti tam praahuh kshetrajna iti tadvidah // 13.2 //

Sri Bhagavan said
    This body, O Kaunteya, is  called the Field; he who knows it (body) is called the Knower of the Field by  those who know of them i.e. by the sages.

This body is the Field:
  The use of the demonstrative pronoun  ‘this’ while referring to the body suggests that this body is different from  the one who perceives or knows it. The word ‘kshetra’ signifies both  ‘body’ and matter. The body is called ‘field’ because the fruits of action are  reaped in it as in the field or it is subject to constant decay. It is the body  in which events happen; all growth, decline and death take place in it. Just as  seeds sown in a field yield the corresponding crops in course of time, even so  seeds of karma sown in this body yield their fruit at the appropriate time. Hence  the body is called the field, the object.

Knower of the Field:
  The word ‘kshetrajna’ or  knower of the field means the individual soul, which is, in reality one with  the Supreme Soul who is the Subject. The entire range of objective reality, the prakriti, the matter, which is open to knowledge through the equipments of  mind, intellect and senses, are material in their constitution, perishable in  their nature and mutable in their essence.

The conscious Self is wholly  different from the aforesaid material world of objective reality. It is the  knower of the matter consisting of perceptions, feelings and thoughts. Prakriti is unconscious activity and purusha is inactive consciousness. The  conscious principle, inactive and detached, which lies behind all active states  as witness, is the Knower of the Field. It is the Lord of the matter and runs  through it. This is the distinction between consciousness and the objects which  that consciousness observes. Kshetrajna is the light of awareness, the Knower  of all objects and He is neither the embodied mind nor an object in the world.  He is the Supreme Lord, calm and eternal and does not need the use of the  senses and the mind for His witnessing. It is the ‘para prakriti’ or the  higher nature referred to in the 7th Chapter of the Gita wherein Sri  krishna described the two prakritis of the Lord. The lower, apara prakriti, or the field, consists of three gunas and the higher prakriti (the  soul or Jiva when individualized) is the Knower of the Field.

The 13th Chapter  proposes to describe the two prakritis- the Field and the Knower of the Field-  in order to determine finally the nature of the Supreme Lord Himself i.e., the  word ‘tat’ in the mahavakya ‘Tat Twam Asi’.

kshetrajnam chaapi maam viddhi sarvakshetreshu bhaarata
    kshetrakshetrajnayor jnaanam yattajjnaanam matam mama // 13.3 //

You also know Me as the knower  of the Field in all Fields, O Bharata. Only the Knowledge of the Field and its  Knower is considered by Me as true knowledge.

Sri Krishna declares that only  the knowledge of the perishable and inert matter and the nature of the Infinite  and Imperishable Spirit is the True Knowledge.

Kshetra or the field is the matter  consisting of equipments of perception and what is perceived by them.  Kshetrajna is the knower of what is perceived through the instruments of  perception. To distinguish thus the worlds of the subject and the object is what  the Lord tells as true knowledge.
   
  SALIENT  POINTS TO BE DISCUSSED

tat kshetram yaccha yaadrikcha yadvikaari yatashcha yat
    sa cha yo yatprabhaavashcha  tatsamaasena me shrinu // 13.4 //

Hear briefly from Me what the  Field is, what its properties are, what its modifications are, from where it  comes, who its knower is and what His powers are.

Sri Krishna says He is going to explain  what constitutes the Field, its properties, its origin, its modifications i.e.  what its by-products are when it changes its form, what is the knowing  principle in the Field and what are powers of perception, feeling and thought  of the Knower of the Field?

CITING THE AUTHORITIES

rishibhirbahudhaa geetam chhandobhirvividhaih prithak
    brahmasootrapadaishchaiva hetumadbhirvinishchitaih // 13.5 //

All this has been sung by  sages in many and different ways, in various distinctive hymns, and also in  well reasoned and convincing passages indicative of Brahman.

Sri Krishna speaks very highly of  the nature of the Field and the Knower of the Field in order to create interest  in Arjuna. He says what He is going to explain are the very truths that are already  contained in the Vedas, the Upanishads and the Brahmasutras or the  aphorisms of Brahman, later systemized by Badarayana. The Veda hymns are called cchandas or rhythmical utterances. These revelations of the sages are  not in the nature of any commandments but they are logical thoughts, full of reasoning  which are highly convincing.
   
  THE  FIELD IS DESCRIBED

mahaabhootaanyahankaaro buddhiravyaktameva cha
    indriyaani dashaikam cha pancha chendriyagocharaah // 13.6 //

The great elements, egoism  (I-consciousness), intellect and also the unmanifested, the ten senses and the  mind and the five objects of the senses.

icchaa dweshah sukham duhkham sanghaatashchetanaa dhritih
    etat kshetram samaasena savikaaramudaahritam // 13.7 //

Desire, hatred, pleasure,  pain, the aggregate (body), intelligence, and fortitude - this briefly stated,  is the Field together with its modifications.

From here onwards Sri Krishna  starts explaining His promised themes one by one in great details. The above  two verses explain various items constituting together as the Field which was  earlier indicated as the body. A reference may be made to the Chapter 7 Part-1  in this series where a detailed discussion has been given covering all these  concepts.
   
Elements: Five in number viz.  space, air, fire, water and earth. (Ref. Ch.7.4)

Egoism: The sense of `I' ness and  `My' ness or the individuality that arises in our relationship with the world  of objects.

Intellect: The determining  faculty which thinks, discriminates and decides.

Unmanifested: They are the unseen  cause, total vasanas (impressions) which rule the mind and intellect in  determining their activities in the outside world. When these vasanas are  manifested they are seen as the world of objects. (Ref. Ch.7.14)

Ten senses: Five sense organs of  perception viz. ear, skin, eye, tongue and nose and five sense organs of action  viz, hands, feet, mouth, anus and generative organs. These are the channels by  which an individual perceives the stimuli and responds to them.

The one: This stands for the mind  which thinks about the stimuli received from the sense organs and sends forth  the responses after getting the judgment from the intellect.

Five objects of the senses: Each  sense organ perceives only one type of sense object. They are ear -sound, skin  - touch, eye - a form, tongue - taste and nose - smell.

The above 24 items constitute kshetra,  the matter or the gross body or the Field.

Their modifications are  enumerated now.  They are desire, hatred,  pleasure, pain, the assemblage of the body, intelligence, steadfastness etc. In  short, not only the gross body, mind and intellect but also the perceptions  experienced through them, the world of objects, emotions and thoughts are  included in the term `Field' - this body. All the world of objects, which  includes emotions and thoughts, are `knowable' put together in a bunch. This is  called the Field, the object. The Knowing Principle or the Knower is the  subject. Real knowledge consists in understanding the distinction between the  object and the subject.

The following five verses  enumerate the 22 qualities which together indicate the Knower of the Field.