Galen divided the functions of the human organism into three large systems which were centred in the liver, the heart, and the brain. The main lines of the theory are as follows. Like Aristotle, Galen conceives things as composed of the four elements, each of which has two of the qualities of hot, cold, dry, and moist in various combinations. They exist in the bodies of animals in the form of the four humours: blood (hot, moist), yellow bile (hot, dry), black bile (cold, dry), and phlegm (cold, moist) and thei