Abstract A common diagnostic for distinguishing between arguments and adjuncts is obligatoriness/optionality : as a rule of thumb, arguments are obligatory and adjuncts are optional. However, there are many examples of optional arguments, which have led researchers to question the usefulness of this diagnostic and sometimes even the very distinction between arguments and adjuncts. This paper aims to show that arguments are not simply optional; they are omissible only under identifiable grammatical and pragmatic conditions. By contrast, there are no conditions on when adjuncts can be omitted. There are instead pragmatic conditions that dictate the inclusion of adjuncts.