"Galileo on the Distinction between Primary and Secondary Qualities (I). A Superessentialist Argument" Galileo’s distinction between primary and secondary qualities has hitherto been examined almost exclusively from a contextual, historical perspective. This paper, the first of two planned, aims to fill this gap by providing a systematic, theoretical analysis of his principal argument for the distinction, as advanced in "The Assayer". I begin with a reconstruction of the key steps in the argument, and then proceed to identify and discuss the (...) epistemological and ontological presuppositions underpinning it. I argue that the most crucial and original assumption behind Galileo’s reasoning is the superessentialist principle, implying the mutual conditionality of the essentiality and reality of a material body’s properties. An in-depth analysis of the assumptions underlying Galileo’s reasoning paves the way for further examination of the most controversial aspect of his theory, that is, the ontological status of the secondary qualities. This issue will be addressed in the second forthcoming paper. (shrink)