Many philosophers who defend a Humean account of laws of nature often endorse scientific realism as well, such as David Lewis and Barry Loewer. It appears that, to them, an account of laws and scientific (anti-)realism are simply separate issues. They think there is nothing inconsistent about being both a Humean and a scientific realist. This presentation calls into question the tenability of this position and takes a close examination of the relation between the Humean accounts of laws and scientific realism. I argue that two of the most prominent Humean accounts of laws, Lewis’ Best System Account (BSA) and Loewer’s Package Deal Account (PDA) are at odds with scientific realism. More specifically, I argue that the PDA leads to scientific anti-realism, and that the BSA does not conform to scientific practice.

The special feature of the PDA is that it identifies the laws of nature and the fundamental ontology together as a package. As a consequence, the laws and the fundamental physical objects have the same metaphysical status: either we regard both of them as objective, or neither of them objective. I distinguish two notions of objectivity: weakly objective and strongly objective. Their main difference is that there could be something that seems to be objective to us, human beings (thus, weakly objective), but ceases to be so if we take into consideration factors that are not shared by other creatures. Scientific realism takes theoretical objects to be strongly objective: there must be a unique set of fundamental physical objects--Martians must reach the same set of fundamental objects as we do, even if they, say, have a different cognitive capacity. The PDA-laws are only weakly objective, but not strongly objective. That is to say, if Martians have a different cognitive capacity, their PDA-laws would be different from our PDA-laws. Since the fundamental physical objects and the laws of nature have the same epistemic status in the PDA, that means: if the PDA-laws are not strongly objective, the fundamental physical objects in the PDA are not strongly objective either. Consequently, Martians might identify a different set of fundamental physical objects. Since scientific realism accepts only a unique set of fundamental physical objects, the PDA is at odds with scientific realism.

Now consider the BSA. It, unlike the PDA, does not identify the laws of nature and the fundamental ontology as a package deal. Instead, it takes the Humean mosaic (distributions of perfectly natural properties) to be metaphysically prior to laws of nature. (After all, the laws are inferred as a way of systematically summarizing the Humean mosaic.) It is this priority relation that does not conform to scientific practice, especially the part of physics that has been conjectured as fundamental. There isn’t anything in scientific practice (or in the theories) that suggests the ontology and its properties are somehow prior to the laws. Rather, when a scientific object is postulated or predicted by a theory, the corresponding laws to which the object is subject are introduced together as a package.