idarh sariratit kaunteya ksetramityabhidhiyate
etadyo vetti tam prahuh ksetrajna iti tadvidah
The Blessed Lord said:
Body pointed out as 'this' (as distinct from the Self), O Kaunteya, is termed as Ksetra, the field and he, who knows it, as such, is called, Ksetrajna, by the sages. 1
Idarir 'sariram Kaunteya Ksetramityabhidhiyate:- A man addresses all the material objects as 'this beast', 'this bird', 'this tree' etc., (meaning thereby that they are distinctly separate from him), but sometimes he calls this body as 'I' or sometimes as 'My'. The fact is that the body which is said to be 'I' or 'My' is also 'this' i.e., separate from the Self. Gross, subtle and causal—all the three bodies are to be called as 'this' or 'these'.
The gross (physical) body, consists of five elements—earth, water, fire, air and ether. It is made of mother's ovum and father's sperm. It is also called 'Annamayakota because it is born of evolute of food and is sustained by it. It is known as 'Idam' (this), because it is known by senses.
The subtle body, consists of five sense-organs, five organs of action, five life-breaths, mind and intellect. It is called 'Pranamayakosa' (sheath of life-breath), because of the predominance of life-breath, 'Manomayakoia' because of the predominance of mind and 'Vijnanamayakosa', because of the predominance of intellect. This subtle body, is also called 'Idam', because it is known by the inner sense.
Ignorance, is called the causal body. A man's knowledge can have an access upto intellect only. Whatever is beyond intellect, is not open to knowledge. So, it is called ignorance. This ignorance being the cause of all bodies is called causal body - 'Ajnanamevasya hi mulakaranam' (Adhyatma., Uttara. 519).This causal body, is also called personal nature and also 'Anandamayakosa (sheath of bliss).