Does similarity exist as a relation between entities ‘out there’, such as a source text and its translation? Or is similarity a matter of perception, a relation ‘in the mind’? What does similarity mean, and what might it imply for translation studies? The article examines the problem of similarity assessment, with initial reference to similarity across different semiotic systems such as music, pictures, and language. It then discusses the relevance to translation studies of the distinction between convergent and divergent similarity. Similarity assessment is shown to depend on point of view, on the contextual situation, on the relation of relevance, and on the purpose of the assessment. It therefore follows that no theory of equivalence can be tenable if it does not include notions of skopos, relevance and context, as well as the subjectivity of the translator. Finally, since extensional definitions of concepts are also based on similarity assessments, the concept of translation itself is also a relative one, perhaps prototypical.