The Function of “Property” in the Bipartite Logic: A Critique of Tusi’s Reading of Avicenna’s Text

Avicenna in al-Isharat says that the “best property” is that which: (1) include all individuals of the subject; (2) is absolute (nafsi) and not relative (nisbi); (3) is inseparable not separable. Again, he believes that “the most useful property for definition” is that which belongs to the subject self-evidently – in addition to satisfying the three above-mentioned cases. The property, on Tusi’s opinion, can be considered from two viewpoints: (1) in itself; (2) due to its being settled in the definition. The best property, from the first viewpoint, is that which satisfies the first three conditions, and from the second viewpoint, is that which satisfies all of the four. It seems that Tusi interprets Avicenna’s text appropriate to Topics (Dialectic), whereas we can propose a better interpretation by means of a systematical analysis on the basis of the bipartite logic as follows: The property is one of the Quinque Voces (Five Universals), which all have a preparing function; and every preparing factor in the bipartite logic is a preparation either to definition or to reasoning. So, in the bipartite logic, if the property is a preparation to definition, it should satisfy all of the four conditions; and if it is a preparation to reasoning, it should satisfy the first three conditions. The aim of the present article is to analyze this topic in Avicenna's logical heritage.