  The Istanbul manuscript also includes one of the few copies of al-Kindi’s On the Intellect to survive in Arabic (it is also preserved in Latin translation). This is the first treatise in the Arabic tradition to give a taxonomy of the types of intellect, such as will become familiar in al-Farabi, Avicenna and Averroes. Other works shed further light on al-Kindi’s psychology (i.e. theory of soul): the Discourse on the Soul consists of supposed quotations from Greek philosophers, That There are Separate Substances uses Aristotle’s Categories to prove that the soul is immaterial, and On Sleep and Dream gives an account of prophetic dreams in terms of Aristotle’s theory of the imagination. Related to al-Kindi’s psychological theories is his only significant surviving work on ethics, On Dispelling Sorrows. (He also composed a collection of ethical anecdotes and sayings ascribed to Socrates, for which see Fakhry 1963.)