The Principle of “Existence Distinction from Essential Substance”: A Comparative Study of the Views of Avicenna and Thomas Aquinas

The essential substance (nature) by itself only consists of its own essence according to the principle of “existence distinction from nature”, which is considered among the most important principles of Avicenna’s metaphysics and the metaphysical element of existence in order to exist must be occurred on it from the outside and from the determinant cause. Ibn Sīnā (Arabic: ابن سینا, Avicenna) also, based on this principle, establishes the important principle of “the necessary existent distinction from the potential existent” and he explains the issue of creation and the proof of existence of God on the basis of these two principles. Some of Avicenna’s phrases in explaining the distinction of existence on nature have led to different interpretations of his word. Ibn Rushd (Arabic: ابن رشد, Averroes)’ reading is one of the most important of these readings which has become popular among medieval thinkers through translating his works and his followers. Avicenna considers the distinction of nature and existence to be external according to Averroes’ view and believes that existence, like accidental categories, affects the nature. However, attention in Avicenna’s phrases shows that his purpose of considering ascription of existence to subject as accidental means that existence is not a part of essence and essentials and it is occurred from external over essence and nature and not that the existence is occurred to the nature like categorical accidents. Thomas Aquinas, though influenced by Averroes, considers his principle of distinction to be different from Avicenna’s and in some minor cases is different from Avicenna but the examination of his works shows that he is influenced by Avicenna in the main framework of the principle of distinction and its explanation is not significantly different from Avicenna’s.