My interpretation (and some verbatim) of Analytical Knowledge of Sankhya Philosophy as given by Krishna to Arjun:
Happiness and sorrow are a product of sense perception and are transient in nature. The person who is not disturbed by happiness and distress and is steady in both is certainly eligible for liberation.
The soul is eternal. Soul is unborn, eternal, ever-existing, undying, immutable and primeval. It accepts new material body and gives up the old and useless ones.
The individual soul is unbreakable, invisible, inconceivable, immutable, insoluble, everlasting, all-pervading, unchangeable, immovable and eternally the same. Krishna asks Arjun not to grieve for the body.
Krishna tells Arjun that as a Ksatriya, he should fight on religious principles and there is no need for hesitation. He urges Arjun to fight for sake of fighting, without considering happiness or distress, loss or gain, victory or defeat -and by doing so he will never incur sin. The lesson here for us is that we should do our karma without consideration of happiness, distress, victory or defeat.