I heard Abu Al-Hassan (asws) saying: ‘The body is built upon four, so from it is the air without which ‘the self’ cannot survive except by its (in and out) flow, and it takes out the diseases which are in the body and the decay, and the dirt (earth) which gives causes the dryness, and the energy (heat), and the food and from it the blood is generated. Indeed! Have you not seen that it (food) goes into the stomach, which digests it until it becomes soft, then cleans it? So the natural process takes the clean blood, then the residue descends along with the water, and it gives birth to the phlegm’.