[22].Surah Al-Hajj [The Pilgrimage]: Ayat 45-46

Ayat 45. And many a township have We destroyed while it was given to wrong-doing, so that it lies in ruins (up to this day), and (many) a deserted well and lofty castles!

Ayat 46. Have they not travelled through the land, and have they hearts wherewith to understand and ears wherewith to hear? Verily, it is not the eyes that grow blind, but it is the hearts which are in the breasts that grow blind.

[Tafseer] How many populations have We destroyed, which were given to wrong- doing? They tumbled down on their roofs. And how many wells are lying idle and neglected, and castles lofty and well-built? The roofs fell in first, and the whole structure, walls and all, came tumbling after, as happens in ruins. The place was turned upside down. In a dry country like Arabia, a well stands as a symbol for a living, flourishing population, and many place-names mean "the well of so-and-so" e.g., Bir 'Ali, a village just south of Madinah, the quality of whose drinking water is  famous, or Abyar Ibn Hassan, a noted stopping place on the road from Makkah to Madinah, about 92 miles from Madinah. Do they not travel through the land, so that their hearts (and minds) may thus learn wisdom and their ears may thus learn to hear? Truly it is not their eyes that are blind, but their hearts which are in their breasts. The word for "heart" in Arabic speech imports both the seat of intelligent faculties and understanding as well as the seat of affections and emotions. Those who reject Allah's Message may have their physical eyes and ears, but their hearts are blind and deaf. If their faculties of understanding were active, would they not  see the Signs of Allah's Providence and Allah's Wrath in nature around them and in the cities and ruins if they travel intelligently? 

Hazrat An-Nu'man bin Bashir reported: Messenger of Allah [SAWW](PBUH) said "Verily, there is a piece of flesh in the body, if it is healthy, the whole body is healthy, and if it is corrupt, the whole body is corrupt. Verily, it is the heart.''

[Muslim Book 10, Chapter 41, Hadith # 3882]. 

Lesson : as mentioned above in Surah Al-Hajj Ayat 46."but it is the hearts which are in the breasts that grow blind" As in Quran and this Hadith specially concerns the condition of ones heart. Its purity or impurity directly affects the human conduct in a good or bad way. It is, therefore, absolutely necessary to cleanse the heart of all wiles and impurities, and this is not possible without sound Faith and consciousness of Allah.