Bhagavad-gita 2.26 - 2.28 Lord Krsna's logic to make Arjuna to stop lamenting and engage in discharging duty

After Arjuna declined to fight in the battlefield of Kurukshetra because of his bewilderment, Lord Krsna explained that there was no need to lament since all living entities are eternal spirit souls which are eternal, unchangeable and unable to be destroyed.

Lord Krsna then provided logic and counter arguments:-
1. Even if someone considers the soul to be always born and die forever, there was no reason to lament. There are philosophers like lokayatika, vaibhasika, nihilistic non-devotional Buddhists, anthropologists and modern material scientists who maintain that life symptoms take place at a certain mature condition of material combination and by the interaction of the physical and chemical elements. No one laments the loss of a certain bulk of chemicals and stops discharging his prescribed duty.

2. If someone believes that the soul or atma vanishes along with the deterioration of the body, then there was no reason to be afraid of being affected by sinful reactions due to his killing his grandfather and teacher. According to this theory, since there are so many living entities generating out of matter every moment, and so many living entities being vanquished every moment, there is no need to grieve for someone's death. Although unable to prove their theory, these philosophers have no faculty to understand the soul. Hence Lord Krsna even used this material conjecture to explain His conclusion.  

3. All created beings are unmanifest in their beginning, manifest in their interim state, and unmanifest again when annihilated. The material existence remains unmanifested before creation eg. a big skyscraper manifests from the earth. Before it's construction it was not manifest and after it is destroyed, it again becomes unmanifest. This fact is true whether one is a believer in the existence of the soul or an atheist. Since all elements remain unmanifested in beginning and at the end, there is no reason to lament for any entity either in the stage of manifestation or unmanifestation.

4. jatasya hi dhruvo mrtyur dhruvam janma mrtasya ca - One has to take birth according to one's activities of life and after finishing one term of activities, one has to die. Also, after death one is sure to take birth again. In this way one is going through cycle of birth and death. By avoiding the discharge of proper duty, Arjuna would not stop the death of his relatives. Instead he would be degraded due to his selection of the wrong path of action.

 Lord Krsna addressed Arjuna as maha-bahu, mighty-armed and behooved him to continue to follow the Vedic principles as a ksatriya. Hence Arjuna need not be afraid or aggrieved at the death of his relatives since he was discharging his proper duty. There was no reason to be subjected to the reactions of sinful acts while carrying out prescribed duties under the instructions of the guru or spiritual master (Lord Krsna).

Bhagavad-gita 2.23 - 2.25 Individual Soul is eternal, unbreakable, unchangeable, cannot be burnt, moistened or withered and present everywhere

The individual soul is invisible and inconceivable because the soul cannot be seen even by the most powerful microscope being so small in magnitude. The soul is eternally the atomic particle of the Spirit whole and remains the same spiritual atom eternally, without change.

The soul is unbreakable and insoluble. Spirit soul can never be cut to pieces by any weapon, nor burned by fire, nor moistened by water, nor dried by heat, nor withered by the wind. 

Weapons may be made of earth elements like swords, based on fire element like firearms, flame weapons or nuclear weapons, water weapons or even tornado weapons based on wind power but they cannot kill the spirit soul. Modern scientists know about fire weapons but have limited knowledge of water or tornado weapons. Nonetheless, the soul can never be cut into pieces, nor annihilated by any number of weapons, regardless of scientific devices.

The individual soul is everlasting and unchangeable, immovable, immutable and eternally the same. Because of ignorance, one cannot explain how the individual soul came into existence. By revealed knowledge, one understands that individual souls are eternally separated parts of the Supreme Soul. In the Varaha Purana, the living entities are described as separated parts and parcels of the Supreme. Individual souls are prone to be covered by the illusory energy and become separated from the association of the Supreme Lord.

Individual souls are like the sparks of a fire. The sparks have the same quality as the fire although limited in quantity. They are prone to be extinguished when out of the fire. Separated individual souls have similar quality as the Supreme Soul although limited in quantity. Being unchangeable, the infinitesimal soul can never become equal to the infinite Supreme Soul.

Living entities are sarva-gata and present everywhere all over the universe. They live on land, in the water, in the air, within the earth and even within fire. Living entities are also present in the sun planet with suitable bodies made of fire to live there.  

According to Bhagavad-gita, the individual souls are eternally separate entities. Even after being liberated from illusion, the living entity retains a separate identity. The existence of the soul and the qualifications of the soul can be established from the basis of sruti or Vedic wisdom. The superior authority of Vedas is like the authority of the mother to identify one's father.