 Beyond these five external senses are the four internal senses that use the brain as their organ. Following the lead of  Ibn Sīnā  (Avicenna), Aquinas puts considerable weight on these sensory powers to explain the sophisticated behavior of animals. In contrast to the consensus over the number of external senses, there had been a great deal of historical disagreement over how many internal senses there are. Aquinas settles on this list of four: