In plain words
Which is joy, which is sorrow?
Hardship and happiness, the two, are joined together; no one has the power to part them; happiness we never see anywhere on its own; when hardship bears fruit, they call it happiness.
When a child is born we call it joy. When it dies we grieve. The cause of the sorrow and of the joy is the one child! When the sun rises we call it day. When the sun sets we call it night. There is not one sun for the night and another sun for the day. There is only one! The nature of your mind itself is the cause of joy and sorrow. The mind is only one. We must welcome both and develop oneness. Through such equanimity, to believe in divinity, to love God, to purify the heart and to carry out one's duties: this is man's chief karma.