Bhagavad Gita, Chapter 13, Verse 2

The Deity said: This body, O son of Kunti, is called Kshetra, and he who knows it is called Kshetrajna by those who know of them (Kshetra and Kshetrajna).

A basic, real distinction gets drawn here first, before anything more complicated follows.

Naming the body as the field and the one aware of it as the knower separates two things that usually get treated as one. Real clarity starts with this simple split.