It is often claimed that principles of individuation imply essential properties of the things individuated. For example, sets are individuated by their members, hence sets have their members essentially. But how does this inference work? First I discuss the form of such inferences, and conclude that the essentialist inference is not a purely formal matter: although there is a form which all principles of individuation have in common, it is not true that any statement of that form is a principle (...) of individuation, and hence there is no valid inference based purely on that form. However, I argue that there is a viable version of the essentialist inference nonetheless. The resources for a proper reconstruction of this type of inference can be gathered from reflections upon the role principles of individuation play in the project of the ontologist: such principles turn out to carry a modal weight similar to that of definitions or conceptual truths. It follows that, no matter how austere the ontology, some portion of essentialist modality is inevitable. (shrink)