Investigating the relationship between two basic notions of existence and thingness (or being and thing) is among the important issues in metaphysical systems. The relationship assumed by a philosopher between these two notions, based on special foundations is effective on his interpretation of the formation of world’s things and the metaphysical condition of non-existents. In the present article, two views of Ibn Sina and Nathan Salmon on the relationship between existence and thingness are presented, and since Ibn Sina’s view is considered as a reaction to Muslim theologians’ view, we briefly mention their views as well. Centuries ago the Muslim theologians, and Salmon in the contemporary times- each on different bases and with various motivations- believed that the relationship between existence and things is 'absolute generality and peculiarity', while Ibn Sina believes in the concomitance of this relationship. Considering the theoretical framework of the discussion, the present article seeks to benefit from the contemporary analytical philosophy’s explanation about the relationship between existence and thingness to better understand Avicenna’s view. More specifically, we are seeking, after comparing the similarities and differences of the views of the two philosophers, to clarify the status of Avicenna’s view. As a secondary goal, we mention one of the most important implications of the discussion (about the relationship between existence and thing), i.e. the metaphysical status of non-existents and the different solutions of the two philosophers for the problem of describing non-existents.