section_path: Principal works/The Book of Healing/Physics
section_name: Physics
paragraph_text: Physics
  
  In mechanics, Avicenna, in The Book of Healing, developed a theory of motion, in which he made a distinction between the inclination (tendency to motion) and force of a projectile, and concluded that motion was a result of an inclination (mayl) transferred to the projectile by the thrower, and that projectile motion in a vacuum would not cease. He viewed inclination as a permanent force whose effect is dissipated by external forces such as air resistance.
  
  The theory of motion presented by Avicenna was probably influenced by the 6th-century Alexandrian scholar John Philoponus. Avicenna's is a less sophisticated variant of the theory of impetus developed by Buridan in the 14th century. It is unclear if Buridan was influenced by Avicenna, or by Philoponus directly.
  
  In optics, Avicenna was among those who argued that light had a speed, observing that "if the perception of light is due to the emission of some sort of particles by a luminous source, the speed of light must be finite." He also provided a wrong explanation of the rainbow phenomenon. Carl Benjamin Boyer described Avicenna's ("Ibn Sīnā") theory on the rainbow as follows:
  
  Independent observation had demonstrated to him that the bow is not formed in the dark cloud but rather in the very thin mist lying between the cloud and the sun or observer. The cloud, he thought, serves as the background of this thin substance, much as a quicksilver lining is placed upon the rear surface of the glass in a mirror. Ibn Sīnā would change the place not only of the bow, but also of the color formation, holding the iridescence to be merely a subjective sensation in the eye.
  
  In 1253, a Latin text entitled Speculum Tripartitum stated the following regarding Avicenna's theory on heat:
  
  Avicenna says in his book of heaven and earth, that heat is generated from motion in external things.