thathaa Dehaantarapraapthirdheerstatra na Muhyati ||
Lord Krishna is stressing, first and foremost on dis-identification with the body and its changes. You are not the body;
Second but simultaneous is the fact that you are the in-dweller of the body and will experience all the changes in the body; but you remain unaffected by it. You are not born, you do not grow, you do not degenerate, you do not become Old and you do not die. All these happen to the body ; when the body falls, you move on to the next body.
The third factor is, while all these changes happen to the body, you as in-dweller, will also experience all of them; But, you are only a witness to all of them; they are not happening to you as the in-dweller. Therefore, a wise man does not grieve for all these changes happening in the body.
Aaagmaapaayinonityaah taamstitikshasva Bhaarata ||
Lord Krishna goes one step further and points out how the sensations arise. Even as in-dweller, we are experiencing the sensations arising in the body. These sensations arise when the sense organs come in contact with the sense objects and carry the inputs received by them, to the brain centres concerned. There, the sense-centers that exist are called tanmatras or matras. These are extremely fine, almost invisible centres that receive the sense organ inputs which come from contact with the external sense objects.
For example, the tanmatra relating to eye-sight, which resides in the brain receives inputs from the external eye-organ through optical nerves, which the external eye receives when it comes in contact with external objects (called Drusyam).
Ultimately the tanmatras decode the information relating to the Drusyam (external Object sighted) to the in-dweller, who experiences it; or witnesses it. The in-dweller is called Drasta, the seer.
Similar process happens in respect of smell, taste, touch and hearing. In all these sensations, the tanmatras come in contact with external objects (through external sense organs and related nerves) , decipher the information and present before the Drasta or the in-dweller.
Some facts about these sensations being pushed before the indweller by the tanmatras are now being mentioned by Lord Krishna.
First, each sensation comes and goes. It cannot remain before the in-dweller forever. It is so with every sensation. All sensations we experience are temporary and fleeting.
The sensations of cold and heat remain as long as the skin remains near ice or fire. Once the skin is away from these external objects, the sensations of cold and heat also recede from us.
Sukha and Duhkha (Pleasure and pain/sorrow) are derived sensations. it is the mind which manufactures them. External object, only creates sight, sound,smell, taste or touch sensations. These sensations are mixed with internal memories stored by the tanmatras in the brain and the final product becomes the Pleasure or Sorrow or Pain sensation.
Pleasure or Sorrow can be produced even without any contact with external objects in the present. The stored memories themselves can re-appear to produce these derived sensations. In any case, every time, they appear, they will also disappear. We cannot hold on to them. But, human desire is - we want to hold on to pleasures. we want to avoid sorrows and pains.
Lord Krishna says - no, this is not possible. you cannot hold on to them. they will come and they will go. Therefore, just live them and bear them.
Samaduhkhasukham Dheeram somrutatvaaya Kalpate ||