BG Chap 13 - The field and its knower

The body, along with its senses and prāṇa is the field, because it is the basis for sprouting the tree of repeated births, by giving the experiences of happiness and distress for the jīva.

He who knows the body in terms of “I and mine” in the conditioned state due to his attachment to the body, and who, being devoid of the conception of “I and mine” in the liberated state knows that he is not attached to the body is the knower of the field. The knower of the field does not denote any jīva within a body, but the jīva who is aware of the difference between the jīva and body. He is a person who, while sleeping, eating and eating performing other activities, knows that this body is not the ātmā, though it is perceived by the ignorant people as themselves; and who knows that the body is the means of both enjoyment and liberation for the ātmā.
The conditioned jīvas eat one fruit of the tree of saṁsāra called distress because the tree has the ability to generate distress even in ripening into svarga. The liberated jīvas eat another fruit called happiness because the tree has the ability to generate happiness in the form of liberation. Thus the one tree of saṁsāra has many forms since it has the capacity to let one attain hell, svarga and liberation. He who knows this tree with the help of the worshipable gurus knows the Vedas. Those who think that the body is the self are not knowers of the field, since they do not know the real nature and function of the body.