 The conditions on this function differ according to different accounts; however, the function is characterized solely in structural terms. It is supposed to capture a relevant similarity relation between the respective theories, such as a structural isomorphism (Suppes 1967; Sneed 1971). Moulines’ criticism that this picture of reduction lacks an ontological link to rule out arbitrary reductions (Moulines 1984) reflects the idea that bridge-laws shouldn’t be given a purely structural interpretation.