The Sankhyayoga in the Gita.

1. In the previous chapter we saw that seeing his friends and relatives having gathered in Kurukshetra to fight left Arjun bewildered and in a state of deep agony. His heart was filled with disappointment. The ruthless war now appeared to Arjun useless. He felt that fighting with his kith and kin for the sake of kingdom will, by no means make him happy. War to him now appeared to be a futile engagement. Despite being a Kshatriya he found begging a better option than killing his near and dear ones in war.
Arjun now, at the spur of that critical moment when war was just to begin on the ground, chose to retreat and shun killing his siblings, elders, friend and relatives that the battle field of Kurukshetra happened to be composed of. Therefore, he began to seek refuge in devising reasons for the same. He told Lord that he would simply not fight as he didn't see any good in this sort of carnage in the name of kingdom. Arjun further argued what glory or satisfaction would accrue to him by killing his own blood of brothers, elders like grandfather Bhishma or the Reverend Guru like Drona? Now begging or mendicancy appeared to him a better option than killing his own blood in the war. Arjun started explaining to the Lord that war was only a wasteful option and mass carnage associated with war serves ultimately no good to anyone, rather is the cause of doom of the whole clan in more ways than one. Arjun went on to say to the Lord that ravages and loss of lives during war would leaves women to the state destitution. Downslide of them in absence of their protecting husbands having been Killed in the battle would give rise to birth of hybrid progeny making the entire clan impure and vitiated for all future to come.
2. Now pause for a while.
Is war good? We shall be saying--No sane mind will advise to go for it.
Is killing one's kith and kin good? No, not at all. Pat will come the reply.
Are not the women the worst sufferer of any war fought? Yes, of course; will be our answer.
Does not Arjun appear to be correct in holding such arguments?
What Arjun said was not incorrect but definitely out of context. It was the attachment of Arjun for their near and dear ones and not his conviction that made him speak so. Had these been born out of his conviction he would not have fought so many battles before with ruthless valour. These arguments were solely to avoid killings of his friends and relatives so dear to him otherwise. This situation was altogether a new experience to the bravest of braves that Arjuna was. Hence these arguments which was against the basic nature of a Kshatriya.
From above it is clear that Arjun was overpowered by the emotions born out of Kutumbprem(weakness of relationship). He did not want to see their kith and kin dying before his own eye by his own bow and arrow. He shivered at the consequences of the devastation. All his pedantic exposition highlighting the futility of war was born out of this weaknesses and though being correct in their own place had no relevance in so far any war is thrust upon or contracted for any purpose whatsoever. Is not it? It was out of emotions which made Arjun to think or argue like that.
The Lord was quick to recognise that Arjun talking only irrelevance. These misgivings born out of ignorance called Moha were neither worthy of his character nor appropriate to the Occasion. Hence we find that in the whole of the discourse the Lord never touched those issues what were raised by Arjun in defence of his disinclination to fight. Rather Arjun was derisively asked by The Lord as to what made him so irresolute that he now intended to deviate from his duty which was simply to fight and nothing short of that.
The Lord said, Arjun you talk like a person of wisdom but in reality it is your attachment which has laid curtain on your intellect. Your attachment has  arrested your Vyavsayatmika Budhhi. This has made you out of balance.
3. The Sankhyayoga as expounded in the Gita
Sankhya as expounded in the Gita is not to be compared to that Sankhya which is referred to as one of the branches of Indian philosophy.
Both are different.
However the Sankhya of Gita, though never defined or explained save a mere mention of it at the foot of Chapter 2 in the closing verse.  The closing verse found at the foot of each of the chapter in the Gita is called Pushpika. Each Pushpika defines the Chapter above it as a separate kind of the Yoga. Thus Pushpika of Chapter 2 defines the subject dealt under that chapter as Sankhyayoga, in absence of it we would not have been in a position to say with evidence that what is described in the second chapter of the Gita is Sankhyayoga of the Gita. According to some Pushpika is not the part of the Gita, but this is not our subject of discussion. The Sankhyayoga, according to what I have gained through reading of the text, is achieving a state of being where all organs of the body despite being allowed to come in contact with their respective subject the latter is unable to make the organ its captive. It is possible in such a way that those organs of the body are simply doing their duty that is performing function assigned to them; nothing more than that. This involves keeping one's eleventh sense that is our mind always in a state of dynamic equilibrium. By dynamic equilibrium we want us to know that situations in our life keep on changing and so should be our response to that change without being swayed by the said change. This can happen only when we are on the guard that our mind does not get merged in the situations whether favourable or unfavourable. Here we are supposed to make use of our intellect to reign our mind. The Sankhyayoga stands to mean that it is a state of being where mind is only the experiencer of the change and in no case it is allowed to become the consumer of the outcome of the effect of the change. 
This is Sankhyayoga of the Gita.
But Sankhyayoga is not only what is barely outlined in above paragraph. It, amongst other experiences, is realising the path of Knowledge of self & soul. It is study of relation of mind, intellect and body. It is knowing subtle interaction of organs (both sense and action) with its respective subject without an element of attachment with the matter with which the organ comes in contact. The sort of contact can be likened lotus flower's petals coming in contact with water(पद्मपत्रमिवाम्भसा). We shall discuss it further elsewhere also according to the context that may follow in due course of this chapters and other ones.
How to keep mind in a perpetual state of dynamic equilibrium has been dealt separately in the Gita as a separate yoga referred to as the Atmasanyamyoga in chapter 6 of the Gita.
4. Immortality of soul and perishable nature of body.
Outwardly it looks strange that the discourse in the Gita should begin with an abstrusly comprehensible character of soul and its other attributes. It also looks bizarre that what was the need of explaining by the Lord to his disciple Arjun the immortality of soul vis a vis perishabilty of body at the battle ground of the Kurukshetra.
How would Arjun have been able to fight and kill his kins unless he was made to realize that fighting was his duty and if that duty involved killing his near and dear ones he must not withdraw. This required on part of him to  understandytruly the real knowledge
I strongly believe that Arjun would not have muster the courage to fight with his kins had Lord not explained to him that he must not remain in self-deception. His
The Lord smiles at Arjun for his wrongful indulgence by way of specious arguments. The Lord asks- O Arjun,! What do you think? Do you think that Me, You and this whole lot of kings have come to exist for the first time? Or is it that you think You, Me and They will not exist hereafter? If you think like that you are under deep illusion and far from reality.
The Lord exhorts Arjun that it is only the body of a person that he, at the best of his bravery, can kill by his bow and arrow not the soul residing in the body. So when soul cannot be destroyed what if the bodies of these people happen to die by your bow and arrow? Death or birth is the attributes of body not of the soul. The soul will seek another body, so why be in grief?
The Bhagwadgita holds out of the question the death of one who happens to come on this earth by way of birth.At the same time it also holds birth of one who dies an inevitable certainty. Thus birth- death-rebirth continues in cycles. The knowledge and appreciation of this fact should not disturb one seeking equanimity of mind. Thus Sankhyayoga truly speaking is the true acquisition of Knowledge and experience of reality that which happens to exist through birth after certain period technically called life comes to an end called death. If you have really acquired this knowledge birth or death will not affect your equanimity of mind.
But how this equanimity of mind to be achieved? - It is possible through cultivating the habit of forgoing the fruit of our Actions. For the simple reasons that it is not within our control. Hence since fruit of an action is not in our control it is useless to develop desire for it. If it accrues well and good if it doesn't it is equally good. This is equanimity. Is it easy without practice? One can easily imagine.
Easier said than done, nevertheless worth practicing. In one word the shortest answer would be that we will have to train our mind and tame our organs like one who is called स्थितप्रज्ञ by the Lord.
The Lord prescribes-
The knowledge of this immortality of soul and perishabilty of body does never mean that one should cease from doing one's duty. Rather the true knowledge and practice of Sankhyayoga enables one to become indifferent to pain and pleasure associated with events impacting us directly or indirectly. That is we reach a stage where unfavourable situation is not able to disturb our balance or irritate us nor favourable circumstances make us mentally stuck to it.
दुखेषु अनुद्विग्न मनः सुखेषु विगतस्पृहः।
वीतराग भय क्रोधः स्थितधीर्मुनिरूच्यते।।
The स्थितप्रज्ञ has complete control over his organs. He Acts just as a tortoise who at once draws all his organs within its hard shell, the moment he feels so to do.

Duty Requires Detachment, otherwise it is not a duty to be called as such.
The beauty of the Gita lies in the fact that even war if thrust upon you for no fault of yours and your all out efforts to avert it notwithstanding should  be fought with a sense of duty what Arjun initially failed to appreciate.
R.R.Prabhakar.
30.09.2019.