the kinetic condition of a material body"—a definition close to that of power in today's physics. Psychology Averroes expounds his thoughts on psychology in his three commentaries on Aristotle's On the Soul. Averroes is interested in explaining the human intellect using philosophical methods and by interpreting Aristotle's ideas. His position on the topic changed throughout his career as his thoughts developed. In his short commentary, the first of the three works, Averroes follows Ibn Bajja's theory that something called the "material intellect" stores specific images that a person encounters. These images serve as basis for the "unification" by the universal "agent intellect", which, once it happens, allow a person to gain universal knowledge about that concept. In his middle commentary, Averroes moves towards the ideas of Al-Farabi and Avicenna, saying the agent intellect gives humans the power of universal understanding, which is the material intellect. Once the person has sufficient empirical encounters with a certain concept, the power activates and gives the person universal knowledge (see also logical induction). In his last commentary—called the Long Commentary—he proposes another theory, which becomes known as the theory of "the unity of the intellect". In it, Averroes argues that there is only one material intellect, which is the same for all humans and is unmixed with human body. To explain how different individuals can have different thoughts, he uses a concept he calls fikr—known as cogitatio in Latin—a process that happens in human brains and contains not universal knowledge but "active consideration of particular things" the person has encountered. This theory attracted controversy when Averroes's works entered Christian Europe; in 1229 Thomas Aquinas wrote a detailed critique titled On the Unity of the Intellect against the Averroists. Medicine While his works in medicine indicate an in-depth theoretical knowledge in medicine of his time, he likely had limited expertise as a practitioner, and declared in one of his works that he had not "practiced much apart from myself, my relatives or my friends." He did serve as a royal physician, but his qualification and education was mostly theoretical. For the most part, Averroes's medical work Al-Kulliyat fi al-Tibb follows the medical doctrine of Galen, an influential Greek physician and author from the second century, which was based on the four humors—blood, yellow bile, black bile, and phlegm, whose balance is necessary for the health of the human body. Averroes's original contributions include his observations