 Goodman (1947) posed another influential problem. Examples  (20a)  and  (20b)  show that the truth-conditions of counterfactuals depend on assumed background facts like the presence of oxygen. However, a moment’s reflection reveals that specifying all of these background facts is quite difficult. The match must be dry, oxygen must be present, wind must be below a certain threshold, the friction between the striking surface and the match must be sufficient to produce heat, that heat must be sufficient to activate the chemical energy stored in the match head, etc. Further, counterfactuals like  (20a)  also rely for their truth on physical laws specific to our world, e.g., the conservation of energy. Goodman’s problem is this: it is difficult to adequately specify these background conditions and laws without further appealing to counterfactuals. This is clearest for laws. As discussed in  §1.3,  some have aimed to distinguish laws from accidental generalizations by noting that only the former support counterfactuals. But if this is a defining feature of laws, and laws are part of the definition of when a counterfactual is true, circularity becomes a concern. Explicit analyses of laws in terms of counterfactuals, like Lange (2009), would make an analysis of counterfactuals in terms of laws circular.