The Mask Fallacy: Why Arguments Based on Property Abstraction Do Not Establish the Original Object

Many philosophical arguments proceed by selecting a limited set of properties from a complex concept, conducting reasoning using only those properties, and then returning the result to the original concept. This paper argues that such reasoning often commits a structural error: the argument proves claims only about an abstract entity defined by the selected properties, not about the original object itself. I call this error the Mask Fallacy. Once an argument replaces an object with an abstraction defined by a subset of its properties, the reasoning becomes logically closed over that abstraction. The conclusion therefore applies only to the masked entity. Returning the result to the original object is unjustified unless the argument is re-established with respect to the full set of properties. The paper formulates this principle formally and shows how it undermines several well-known philosophical arguments.