In Medieval Arabic medical texts, a specific property (khāṣṣa) is thought to be one of the effects of a medicine, and effective in a specific humor or organ. This property is mainly mentioned to explain two phenomena, purgative medicines' attraction of a certain humor and theriacas strengthening of human innate heat. Galen had advocated the theory that the faculty of attracting a specific material inheres in a medical substance as its nature (referred to as the theory of inherence). The same view can be seen in the texts of Islamic philosopher-physicians such as Ibn Sīnā (d. 1037). On the other hand, Ibn Rushd (d. 1198) perceived the defects of this theory and criticised it. This article examines his criticism of the theory of inherence in his discussions about purgative medicines and theriacas.
Ibn Rushd says that using the theory of inheritance, we cannot explain the phenomenon that when someone takes more than one dose of purgative medicine, it attracts not only the specific humor, but all of the humors. He then proposes the alternative theory that the specific property originates in the proportions of the qualities in the attracting and the attracted materials. From this perspective, he insists that the object of attraction varies according to the amount of the heat in the medicine.
As for theriaca, Ibn Rushd criticises the theory of inherence as seen in the writings of Ibn Sīnā Ibn Sīnā claims that theriaca's specific property is generated from its substance, i.e. the combination of form with matter, not the mixture of the four qualities. But according to Ibn Rushd, with this explanation, it is impossible to explain the body's various responses to theriaca. Therefore he maintains that one must explain its specific property in terms of the four qualities.
To conclude, Ibn Rushd considers his theory to be more capable of explaining various phenomena than the theory of inherence is.