For the ancient Mexicas, the composition of the human body was similar to that of the cosmos, with both being composed of dense and light substances. The light substance of the human body was divine in nature and formed the different souls of each human being. Some souls were indispensable for human existence while others were unnecessary and often harmful. The dense part of the human body functioned through its union with the souls. Like the different souls, the dense parts of the human body also had specific functions dedicated to different activities. For example, human thought derived primarily from the heart. Souls could be damaged, which could cause them to malfunction and lead to illness and possibly death in the human being. As the souls were divine, each was a conscious being with its own personality; thus there could be disagreements between them. Disharmony could also lead to illness.