Life is an interplay of energy and matter. It all began with energy and then matter formed subsequently at different stages. The combination of molecules of matter gave rise to different types of compounds with varying properties which resulted in immense diversity. One such combination is the physical bodies of life forms which sustain on the energy coming from the arrangement of matter, known as food. Some life forms (trees) know how to make food from sunlight and others (animals) survive on the food made by them. In a nutshell, life is a game of energy and matter.
This context would help us to understand some words used by Krishna like 𝘽𝙧𝙖𝙝𝙢𝙖, 𝙆𝙖𝙧𝙢𝙖, 𝘼𝙙𝙝𝙮𝙖𝙩𝙢𝙖, 𝘼𝙙𝙝𝙞𝙗𝙝𝙪𝙩𝙖𝙢, 𝘼𝙙𝙝𝙞𝙙𝙖𝙞𝙫𝙖𝙢 and 𝘼𝙙𝙝𝙞𝙮𝙖𝙟𝙣𝙖𝙝 (7.28 and 7.29). Arjun wants to know about them (8.1 and 8.2) and Krishna says, "𝘼𝙙𝙝𝙞𝙗𝙝𝙪𝙩𝙖𝙢 is perishable nature; 𝘼𝙙𝙝𝙞𝙙𝙖𝙞𝙫𝙖𝙢 is the purusha (residing in Pura i.e. city) and I am the 𝘼𝙙𝙝𝙞𝙮𝙖𝙟𝙣𝙖𝙝 here in the body" (8.4).
𝘼𝙙𝙝𝙞𝙗𝙝𝙪𝙩𝙖𝙢 is the matter or 'form' which gets perished in time; 𝘼𝙙𝙝𝙞𝙙𝙖𝙞𝙫𝙖𝙢 is like 'formless' energy present in the body; and 𝘼𝙙𝙝𝙞𝙮𝙖𝙟𝙣𝙖𝙝 makes this play possible like a director of a drama, Krishna being that director.
Krishna said, "Detachment of cosmic energy capable of creation (𝙗𝙝𝙪𝙩𝙖-𝙗𝙝𝙖𝙫𝙖-𝙪𝙙𝙗𝙝𝙖𝙫𝙖-𝙠𝙖𝙧𝙖𝙝 𝙫𝙞𝙨𝙖𝙧𝙜𝙖) is called 𝙆𝙖𝙧𝙢𝙖." Energy is essential for anything to be done and 𝙆𝙖𝙧𝙢𝙖 is the play of energy which brings life to inert matter. Essentially, 𝙆𝙖𝙧𝙢𝙖 is the drawing of energy by life forms (matter) for their sustenance. What we perceive are the effects of this interplay which are known as 𝙠𝙖𝙧𝙢𝙖-𝙥𝙝𝙖𝙡 .
Krishna says '𝙨𝙬𝙖𝙗𝙝𝙖𝙫' (intrinsic nature) is 𝘼𝙙𝙝𝙮𝙖𝙩𝙢𝙖 (8.3). For example, silence is our '𝙨𝙬𝙖𝙗𝙝𝙖𝙫', while languages are acquired. 𝙎𝙬𝙖𝙗𝙝𝙖𝙫 is what remains when we shed the 'borrowed'. While acquiring skills and things, it's about not letting them drive us into comparisons or define who we are.