The distinction of argument and adjunct has been an important notion in linguistic theories, especially at the interface of syntax and semantics, and this distinction has been believed to be binary and categorical. However, there are some gray-area cases such as instruments and participant locations, for which the distinction does not appear to be so clear. In this paper, I will explore whether the argument/adjunct distinction is categorical by examining the controversial cases, particularly the instrumental PPs. I will review the syntactic, semantic, and psychological criteria and evidence that have been proposed in literature and see if instruments can be categorized as either arguments or adjuncts by those criteria. By showing that different criteria yield contradictory results, I conclude that roles like instruments have characteristics of both arguments and adjuncts, which seems to suggest that the argument/adjunct distinction is not categorical.