The Proof of the Truthful (, also translated Demonstration of the Truthful or Proof of the Veracious, among others) is a formal argument for proving the existence of God introduced by the Islamic philosopher Avicenna (also known as Ibn Sina, 9801037). Avicenna argued that there must be a "necessary existent" (), an entity that cannot not exist. The argument says that the entire set of contingent things must have a cause that is not contingent because otherwise it would be included in the set. Furthermore, through a series of arguments, he derived that the necessary existent must have attributes that he identified with God in Islam, including unity, simplicity, immateriality, intellect, power, generosity, and goodness. Historian of philosophy Peter Adamson called the argument one of the most influential medieval arguments for God's existence, and Avicenna's biggest contribution to the history of philosophy. It was enthusiastically received and repeated (sometimes with modification) by later philosophers, including generations of Muslim philosophers, Western Christian philosophers such as Thomas Aquinas and Duns Scotus, and Jewish philosophers such as Maimonides. Critics of the argument include Averroes, who objected to its methodology, Al-Ghazali, who disagreed with its characterization of God, and modern critics who state that its piecemeal derivation of God's attributes allows people to accept parts of the argument but still reject God's existence. There is no consensus among modern scholars on the classification of the argument; some say that it is ontological while others say it is cosmological. Origin The argument is outlined in Avicenna's various works. The most concise and influential form is found in the fourth "class" of his Remarks and Admonitions (Al-isharat wa al-tanbihat). It is also present in BookII, Chapter12 of the Book of Salvation (Kitab al-najat) and throughout the Metaphysics section of the Book of Healing (al-Shifa). The passages in Remarks and Admonitions draw a distinction between two types of proof for the existence of God: the first is derived from reflection on nothing but existence itself; the second requires reflection on things such as God's creations or God's acts. Avicenna says that the first type is the proof for "the truthful", which is more solid and nobler than the second one, which is proof for a certain "group of people". According to the professor of Islamic philosophy Shams C. Inati, by "the truthful" Avicenna means the philosophers, and the "group of people" means the theologians and others who