From Chapter two of the Bhagavad Gita:
Lord Shri Krishna said: "Why grieve for those for whom no grief is due, and yet profess wisdom? The wise grieve neither for the dead nor the living. There was never a time when I was not, nor thou, nor these princes were not; there will never be a time when we shall cease to be. As the soul experiences in this body infancy, youth and old age, so finally it passes into another. The wise have no delusion about this. Those external relations which bring cold and heat, pain and happiness, they come and go; they are not permanent. Endure them bravely, O Prince! The hero whose soul is unmoved by circumstance, who accepts pleasure and pain with equanimity, only he is fit for immortality.
That which is not, shall never be; that which is, shall never cease to be. To the wise, these truths are self-evident. The Spirit, which pervades all that we see, is imperishable. Nothing can destroy the Spirit. The material bodies which this Eternal, Indestructible, Immeasurable Spirit inhabits are all finite. Therefore fight, O Valiant Man! He who thinks that the Spirit kills, and he who thinks of It as killed, are both ignorant. The Spirit kills not, nor is It killed. It was not born; It will never die, nor once having been, can It cease to be. Unborn, Eternal, Ever-enduring, yet Most Ancient, the Spirit dies not when the body is dead. He who knows the Spirit as Indestructible, Immortal, Unborn, Always-the-Same, how should he kill or cause to be killed? As a man discards his threadbare robes and puts on new, so the Spirit throws off Its wornout bodies and takes fresh ones. Weapons cleave It not, fire burns It not, water drenches It not, and wind dries It not. It is impenetrable; It can be neither drowned nor scorched nor dried. It is Eternal, Allpervading, Unchanging, Immovable and Most Ancient. It is named the Unmanifest, the Unthinkable, the immutable. Wherefore, knowing the Spirit as such, thou hast no cause to grieve. Even if thou thinkest of It as constantly being born, constantly dying, even then, O Mighty Man, thou still hast no cause to grieve. For death is as sure for that which is born, as birth is for that which is dead. Therefore grieve not for what is inevitable. The end and the beginning of beings are unknown. We see only the intervening formations. Then what cause is there for grief? One hears of the Spirit with surprise, another thinks It marvellous, the third listens without comprehending. Thus, though many are told about It, scarcely is there one who knows It. Be not anxious about these armies. The Spirit in man is imperishable. Thou must look at thy duty. Nothing can be more welcome to a soldier than a righteous war. Therefore to waver in this resolve is unworthy, O Arjuna! Blessed are the soldiers who find their opportunity. This opportunity has opened for thee the gates of heaven. Refuse to fight in this righteous cause, and thou wilt be a traitor, lost to fame, incurring only sin. Men will talk forever of thy disgrace; and to the noble, dishonour is worse than death.
Great generals will think that thou hast fled from the battlefield through cowardice; though once honoured thou wilt seem despicable. Thine enemies will spread scandal and mock at thy courage. Can anything be more humiliating? If killed, thou shalt attain Heaven; if victorious, enjoy the kingdom of earth. Therefore arise, O Son of Kunti, and fight! Look upon pleasure and pain, victory and defeat, with an equal eye. Make ready for the combat, and thou shalt commit no sin. I have told thee the philosophy of Knowledge. Now listen and I will explain the philosophy of Action, by means of which, O Arjuna, thou shalt break through the bondage of all action. On this Path, endeavour is never wasted, nor can it ever be repressed. Even a very little of its practice protects one from great danger. By its means, the straying intellect becomes steadied in the contemplation of one object only; whereas the minds of the irresolute stray into bypaths innumerable. Only the ignorant speak in figurative language. It is they who extol the letter of the scriptures, saying, `There is nothing deeper than this.’ Consulting only their own desires, they construct their own heaven, devising arduous and complex rites to secure their own pleasure and their own power; and the only result is rebirth. While their minds are absorbed with ideas of power and personal enjoyment, they cannot concentrate their discrimination on one point. The Vedic Scriptures tell of the three constituents of life – the Qualities. Rise above all of them, O Arjuna, above all the pairs of opposing sensations; be steady in truth, free from worldly anxieties and centered in the Self. As a man can drink water from any side of a full tank, so the skilled theologian can wrest from any scripture that which will serve his purpose. But thou hast only the right to work, but none to the fruit thereof. Let not then the fruit of thy action be thy motive; nor yet be thou enamored of inaction. Perform all thy actions with mind concentrated on the Divine, renouncing attachment and looking upon success and failure with an equal eye. Spirituality implies equanimity. Physical action is far inferior to an intellect concentrated on the Divine. Have recourse then to Pure Intelligence. It is only the petty-minded who work for reward. When a man attains to Pure Reason, he renounces in this world the results of good and evil alike. Cling thou to Right Action. Spirituality is the real art of living. The sages guided by Pure Intellect renounce the fruit of action; and, freed from the chains of rebirth, they reach the highest bliss."