What is the relationship of the heart to righteousness and corruption?

The texts of Islamic law indicate two intertwined matters: The soundness or corruption of the heart affects all the limbs, and the heart is influenced by the uprightness or crookedness of the limbs. The first is indicated by the Prophet, peace and blessings be upon him, who said: "Indeed, there is a piece of flesh in the body, if it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt. Indeed, it is the heart." And by the Almighty's saying: "The Day when neither wealth nor sons will benefit, but only he who comes to Allah with a sound heart." The second is indicated by the Almighty's saying: "And when they went astray, Allah made their hearts go astray." And by the Prophet, peace and blessings be upon him, who said: "Do not laugh much, for excessive laughter deadens the heart." The Prophet, peace and blessings be upon him, combined both matters when he said: "Indeed, Allah does not look at your forms or your wealth, but He looks at your hearts and your deeds."