 On Williamson's account we can generally trust our capacity for assessing counterfactual conditionals in imagination, as this is informed and disciplined by a great amount of background knowledge concerning the workings of nature. This background information includes general knowledge of chemical, physical, and other basic scientific facts, as well as some grasp of the causal and natural laws. On any given occasion, we keep some of this background knowledge fixed within the scope of the supposition as we carry out our counterfactual evaluations. Goodman’s (1955) classical problem of cotenability concerns the question of which facts or beliefs are cotenable with the antecedent of the counterfactual conditional. That is, which facts or beliefs are to be held fixed in a counterfactual development? As we will see, many have raised concerns about what to count as cotenable when evaluating a counterfactual supposition. Crucially, background knowledge also involves some grasp of “constitutive” facts, as Williamson sometimes calls them. These apparently include essentialist facts and principles—for example, Williamson mentions Kripke’s principle of the necessity of origins, which holds that the material origins of an entity are essential to it.