Although Galen was the supreme medical authority until the seventeenth century, his views were often extended or modified. An early example of this phenomenon is the addition of a ventricular localization theory of psychological faculties to Galen’s account of the brain. The first theory of this type that we know of was presented by Posidonius of Byzantium (end of the fourth century CE), who said that imagination is due to the forepart of the brain, reason to the middle ventricle, and memory to the hind part of the brain (Aetius 1534, 1549, book 6, ch. 2). A few decades later, Nemesius of Emesa (ca. 400 CE) was more specific and maintained that the anterior ventricle is the organ of imagination, the middle ventricle the organ of reason, and the posterior ventricle the organ of memory (Nemesius 1802, chs. 6–13). The latter theory was almost universally adopted until the middle of the sixteenth century, although there were numerous variants. The most important variant was due to Avicenna (980–1037 CE), who devised it by projecting the psychological distinctions found in Aristotle’s On the soul onto the ventricular system of the brain (Rahman 1952).