One of the most significant results in syntax has been a deep empirical and, to some degree, theoretical understanding of the argument/adjunct distinction, which underlies a range of superficially disparate phenomena. Therefore, any phenomenon that seems to challenge the argument/adjunct distinction merits careful examination. This paper investigates just such a phenomenon: proleptic PPs. Previous claims about the argument/adjunct status of proleptic PPs are contradictory and mostly unsubstantiated. The paper subjects proleptic PPs to argument/adjunct diagnostics and shows that they unambiguously pattern as arguments: they cannot iterate, survive do so–replacement, or be stranded under vP-pseudoclefting; reconstruct for Condition C under vP-preposing; and are L-selected. They also pattern as arguments on a novel argument/adjunct diagnostic developed here, selectional switch: if adding XP to a structure changes the selectional interactions between a head Y and some ZP ≠ XP, then XP is an argument. Finally, the paper considers counterarguments to the view it defends, showing that they are unsuccessful or irrelevant. Thus, even XPs whose argument/adjunct status initially seems murky can turn out on closer scrutiny to pattern unambiguously as one or the other, supporting the traditional but not uncontested view that the argument/adjunct distinction runs deep, and suggesting that it may be categorical.