Ontology is now widely recognised to be of central importance to science, philosophy and the interaction between the two. Yet there is no clear agreement on how it should be done or even what the term means. This paper is concerned with only one form of such theorising, namely the transcendental argument (henceforth ‘TA’). For this mode of realist theorising has attracted significant criticism and misunderstanding such that many continue to deny it any validity at all.