 The thesis that set theory is most suitable for serving as the foundations of mathematics is by no means uncontroversial. Over the past decades, category theory has presented itself as a rival for this role. Category theory is a mathematical theory that was developed in the middle of the twentieth century. Unlike in set theory, in category theory mathematical objects are only defined up to isomorphism. This means that Benacerraf’s identification problem cannot be raised for category theoretical concepts and ‘objects’. At the same time, (roughly) everything that can be done in set theory can be done in category theory (but not always in a natural manner), and vice versa (again not always in a natural manner). This means that for a structuralist perspective, category theory is an attractive candidate for providing the foundations of mathematics (McLarty 2004).