 Second, conceptions of reduction in the philosophy of science, which draw on explanatory notions, are committed to the assumption that there is a tight connection between reduction and ground, conditional on the assumption that explanatory vocabulary (interpreted non-causally) does in fact introduce a notion of ground, as some grounding theorists would have it. A concept of ground is often introduced by reference to explanatory connectives, such as ‘because’, and ‘in virtue of’. On this interpretation, there is a close link to conceptions of reduction in the philosophy of science. Within this strand of the reduction debate, reduction has been conceived of as an explanatory connection (see above). Of course, a robust metaphysical interpretation of explanatory links in terms of ground stands in stark contrast to some standard conceptions of explanation within the philosophy of science.  Realists have mainly focused on causal explanation (Salmon 1989); only recently, philosophers of science have turned to non-causal, mechanistic dependence (cf. Machamer et al. 2000, Craver 2005, 2007). Others have taken explanation to be primarily an epistemic or pragmatic affair (Kitcher 1989, Achinstein 1983, van Fraassen 1980). Nevertheless, it is worth noting that it is an open question whether (non-causal) explanation can be construed pragmatically or epistemically, or whether it requires a metaphysically robust interpretation. If it turns out that explanation does require a metaphysically robust interpretation, the fact that reduction has been construed as an explanatory notion in the philosophy of science may be seen as an indicator that claims of reduction are, properly construed, claims of ground.