 There are three rival suggestions on the market, all of which argue that weaker notions of reduction still serve the purpose of the reductivist: The reductive link could be spelled out in terms of supervenience (Kim 1982, 1990), causation (Enç 1976) and realization (Kim 1998; Melnyk 2003; Shoemaker 2007. Whereas usual stipulations of supervenience do not rule out the possibility that a supervenes on b and, at the same time, b supervenes on a, at least causation seems to be relevantly directional and so do justice to the directionality of reduction. Nevertheless, causation is probably not what is required: the occurrence of water is not caused by the occurrence of H2O (in the usual meaning of the term ‘causation’); there is no temporal component involved, what is, on most accounts, required by causation.