The Extensionality of Parthood and Composition

I focus on three mereological principles: the extensionality of parthood (EP), the uniqueness of composition (UC), and the extensionality of composition (EC). These principles are not equivalent. Nevertheless they are closely related (and often equated), since they all reflect the basic nominalistic dictum ‘No difference without a difference maker'. All of them, individually or collectively, have been challenged on philosophical grounds. In the first part I argue that such challenges do not quite threaten (EP), because they are either self‐defeating or unsupported. In the second part I argue that they do little to undermine the tenability of (UC) and (EC) either.