"Does the soul progress more through sorrow or through the joy in this world?"

"The mind and spirit of man advance when he is tried by suffering. The more the ground is ploughed the better the seed will grow, the better the harvest will be. Just as the plough furrows the earth deeply, purifying it of weeds and thistles, so suffering and tribulation free man from the petty affairs of this worldly life until he arrives at a state of complete detachment. His attitude in this world will be that of divine happiness. Man is, so to speak, unripe: the heat of the fire of suffering will mature him. Look back to the times past and you will find that the greatest men have suffered most."
-‘Abdu’l-Bahá, Paris Talks, p. 178-179

"Thou hast asked Me concerning the nature of the soul."

"Thou hast asked Me concerning the nature of the soul. Know, verily, that the soul is a sign of God, a heavenly gem whose reality the most learned of men hath failed to grasp, and whose mystery no mind, however acute, can ever hope to unravel. It is the first among all created things to declare the excellence of its Creator, the first to recognize His glory, to cleave to His truth, and to bow down in adoration before Him. If it be faithful to God, it will reflect His light, and will, eventually, return unto Him. If it fail, however, in its allegiance to its Creator, it will become a victim to self and passion, and will, in the end, sink in their depths."

- Baha'u'llah, Gleanings from the Writings of Baha'u'llah, p. 158-159