Wilfrid Sellars's contribution to the volume of Library of Living Philosophers on Rudolf Carnap deals with an issue that touches on the most important subject of dialogue between them, namely, the status of abstract entities. This was also a crucial issue for Sellars's research at that time and in the same year an important essay, titled Abstract Entities, also appeared. Sellars came back to these issues many times in these years, because he was working on a nominalistic reinterpretation of the linguistic framework of abstract entities, which was interconnected with his general approach to semantics and related to his naturalistic perspective. The kernel of Sellars's nominalism concerns more than the existence of abstract entities, the idea that they play some role in mental activity. Sellars's philosophical work, on the contrary, aims at giving a better answer and reaching a deeper understanding of the same, old but not meaningless, metaphysical issues. Carnap's philosophy tries to supersede those same traditional philosophical disputes.