Mental fictionalism holds that folk psychology should be regarded as a kind of fiction. The present version gives a Lewisian prefix semantics for mentalistic discourse, where roughly, a mentalistic sentence “p” is true iff “p” is deducible from the folk psychological fiction. An eliminativist version of the view can seem self-refuting, but this charge is neutralized. Yet a different kind of “self-effacing” emerges: Mental fictionalism appears to be a mere “parasite” on a future science of cognition, without contributing anything substantial. The paper then rebuts the objection, illustrating that prefix semantics resolves a lingering problem for eliminativism from Boghossian. The problem is that eliminativists seem unable to adopt realism about neuroscience, for such realism implies that neuroscientific statements *represent* reality accurately. However, a deflationary version of prefix semantics allows the eliminativist to draw an ontologically relevant distinction (roughly) between truths that have a story-telling prefix and those that do not. (Deflationism means there is no implication that the unprefixed sentences robustly represent reality.) The overarching lesson is that eliminativists need to approach to ontology carefully so to avoid self-refutation; however, prefix semantical mental fictionalism provides the resources for them to do so.