Using the notorious bridge law “water is H₂O” and the relation between molecular structure and quantum mechanics as examples, I argue that it doesn’t make sense to aim for specific definition(s) of intertheoretical or interdiscourse relation(s) between chemistry and physics (reduction, supervenience, what have you). Proposed definitions of interdiscourse and part-whole relations are interesting only if they provide insight in the variegated interconnected patchwork of theories and beliefs. There is “automatically” some sort of interdiscourse relation if different discourses claim to have something to say about the same situation (event, system), which is the basis of (contingent) local supervenience relations, which, proper empirically support being provided, can be upgraded to ceteris paribus bridge laws. Because of the ceteris paribus feature, and the discourse dependence of event identification, there is at best only global supervenience of the “special sciences” on the physical (and of parts of physics on other parts of physics).