عَجِبْتُ لِمَنْ يَتَفَكَّرُ فِي مَأْكُولِهِ كَيْفَ لَا يَتَفَكَّرُ فِي مَعْقُولِهِ، فَيُجَنِّبُ بَطْنَهُ مَا يُؤذِيهِ وَيُودِعُ صَدْرَهُ مَا يُرْدِيهِ
Imam Hasan ('a) is narrated to have said: I am surprised at those who reflect on the food that they are going to eat and yet they do not think at all in regards to the food of their soul. They stay away from unhealthy food, but they fill their hearts with various destructive matters.
Just as the Imam ('a) has mentioned, people are very careful in regards to what they eat. They will only eat foods which they trust and if there is a ten percent chance of illness or food poisoning, they will not even think of eating that food. Some people go beyond this and follow all sorts of nutritional precepts and rules which they believe will aid them in regards to their health. In a nutshell, people care deeply about the food that they eat.
Yet when it comes to the food pertaining to their soul, they take in whatever they find, irregardless of how it will affect them. Even if they are about to ingest something which has a 95% chance of illness and the potential to poison their soul, they don’t really pay any attention and they carelessly take it in. If people tell them words of slander and backbiting, they sit and listen without any fear. This is in reality the equivalent of someone placing a plate of spoiled food in front of them, which will certainly make them ill, and their eating of it.
What we must realize is that various things which relate to the spirit naturally have an effect on the spirit, and the spirit grows or diminishes as a result of what it takes in. This is exactly like when we eat food and our bodies either grow strong from it, or grow weak and become ill as a result of it. It is truly amazing that people pay such detailed attention to the foods related to their physical bodies, but are so utterly careless when it comes to the food of their souls.
 Safīnat al-Biḥār, Section on Food.