Physicians and philosophers often argued whether the heart or brain was formed first in the fetus, and this debate forms an important part of Mansur ibn Ilyas' written works. In his works, Mansur ibn Ilyas argues that the heart is the first organ to form, unlike Hippocrates who argued that the brain is the first organ. Mansur ibn Ilyas' reasoning for the heart as the primary organ was that the semen is composed of air and strong heat, creating a substance called pneuma, which needs to be contained or it will decompose. The containment of the pneuma is within the heart, creating the body and making the heart the main source of the body’s natural heat. The heart also acts as the main force in forming the other organs; using the heat from the heart, the heart is able to provide the rest of the body with nourishment. This marks the start of the formation of the liver, which holds the source of nourishment within it. Lastly, Mansur argued that the brain is the organ that contains the senses, and these senses are what gives life force to the body. If the brain is formed first, like Hippocrates said, then there is nothing for the brain to give life to. The heart needs to be formed before the brain, so that the brain can give its life force to the rest of the body. Throughout his works, Mansur made references to the works of Aristotle, Hippocrates, al-Razi, and Avicenna.