He defined medicine ("tibb") as follows:  Avicenna begins part one by dividing theoretical medicine and medical practice. He describes what he says are the "four causes" of illness, based on Aristotelian philosophy: The material cause, the efficient cause, the formal cause, and the final cause:   Avicenna's thesis on the elements of the cosmos is described by Gruner as "the foundation of the whole "Canon"". Avicenna insists here that a physician must assume the four elements that are described by natural philosophy, although Avicenna makes it clear that he distinguishes between the "simple" element, not mixed with anything else, and what we actually experience as water or air, such as the sea or the atmosphere. The elements we experience are mixed with small amounts of other elements and are therefore not the pure elemental substances. The "light" elements are fire and air, while the "heavy" are earth and water:   The "Canon of Medicine" divides the thesis on temperaments into three subsections; a general overview, one based on members of the body, and temperaments based on age. The temperaments are reported to be the interaction between the four different element's qualities, such as the conflict between dryness, wetness, cold, and hot. Avicenna suggests that these qualities battle between each other until an equilibrium state is reached and this state is known as the temperaments. The "Canon" also adopted the ancient theory of Four Temperaments and extended it to encompass "emotional aspects, mental capacity, moral attitudes, self-awareness, movements and dreams". This expanded theory of four temperaments is given in the following table:  Canon describes humans as having eight different "varieties of equipoise", or differing temperaments. The temperaments fall under two categories; In relation to beings other than men and in relation to the individual himself. "the equability of the temperament seen in man as compared with other creatures" ii. the temperament of other human beings iii. external factors "such as race, climate, atmosphere" iv. in relation to extreme climates v. "as compared to another person" vi. comparison of the individual himself vii. comparing one member of the body with another member of the body viii. comparison of a member to itself  "The Canon" continues to explain the sun's position in relation to ideal temperament and the role that climate and human skin play. Organs are nowhere near ideal in temperament, but skin comes the closest.