Defining Soul - verses from Gita

Never was there a time when I did not exist, nor you, nor all these kings; nor in the future shall any of us cease to be.
As the embodied soul continuously passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. A self-realized soul is not bewildered by such a change.
O son of Kunti, the nonpermanent appearance of happiness and distress, and their disappearance in due course, are like the appearance and disappearance of winter and summer seasons. They arise from sense perception, O scion of Bharata, and one must learn to tolerate them without being disturbed.
O best among men [Arjuna], the person who is not disturbed by happiness and distress and is steady in both is certainly eligible for liberation.
Those who are seers of the truth have concluded that of the nonexistent there is no endurance, and of the existent there is no cessation. This seers have concluded by studying the nature of both.
Know that which pervades the entire body is indestructible. No one is able to destroy the imperishable soul.
Only the material body of the indestructible, immeasurable and eternal living entity is subject to destruction; therefore, fight, O descendant of Bharata.
He who thinks that the living entity is the slayer or that he is slain, does not understand. One who is in knowledge knows that the self slays not nor is slain.
For the soul there is never birth nor death. Nor, having once been, does he ever cease to be. He is unborn, eternal, ever-existing, undying and primeval. He is not slain when the body is slain.
O Partha, how can a person who knows that the soul is indestructible, unborn, eternal and immutable, kill anyone or cause anyone to kill?
As a person puts on new garments, giving up old ones, similarly, the soul accepts new material bodies, giving up the old and useless ones.
The soul can never be cut into pieces by any weapon, nor can he be burned by fire, nor moistened by water, nor withered by the wind.
This individual soul is unbreakable and insoluble, and can be neither burned nor dried. He is everlasting, all-pervading, unchangeable, immovable and eternally the same. It is said that the soul is invisible, inconceivable, immutable, and unchangeable. Knowing this, you should not grieve for the body.
If, however, you think that the soul is perpetually born and always dies, still you still have no reason to lament, O mighty-armed.
For one who has taken his birth, death is certain; and for one who is dead, birth is certain. Therefore, in the unavoidable discharge of your duty, you should not lament.
All created beings are unmanifest in their beginning, manifest in their interim state, and unmanifest again when they are annihilated. So what need is there for lamentation?
Some look at the soul as amazing, some describe him as amazing, and some hear of him as amazing, while others, even after hearing about him, cannot understand him at all.
O descendant of Bharata, he who dwells in the body is eternal and can never be slain. Therefore you need not grieve for any creature.