Swami Vivekananda 
Karma Yoga 

The word Karma is derived from the Sanskrit Kri, to do; all action is Karma. Technically, this word also means the effects of actions. In connection with metaphysics, it sometimes means the effects, of which our past actions were the causes. But in Karma-Yoga we have simply to do with the word Karma as meaning work. The goal of mankind is knowledge; that is the one ideal placed before us by Eastern philosophy. Pleasure is not the goal of man, but knowledge. Pleasure and happiness come to an end. It is a mistake to suppose that pleasure is the goal; the cause of all the miseries we have in the world is that men foolishly think pleasure to be the ideal to strive for. After a time man finds that it is not happiness, but knowledge, towards which he is going, and that both pleasure and pain are great teachers, and that he learns as much from evil as from good. As pleasure and pain pass before his soul they leave upon it different pictures, and the result of these combined impressions is what is called man's "character." If you take the character of any man it really is but the aggregate of tendencies, the sum-total of the bent of his mind; you will find that misery and happiness are equal factors in the formation of that character. Good and evil have an equal share in moulding character, and in some instances misery is a greater teacher than happiness. In studying the great characters the world has produced, I dare say, in the vast majority of cases, it would be found that it was misery that taught more than happiness, it was poverty that taught more than wealth, it was blows that brought out their inner fire more than praise.