The notion that "nature" comes equipped with its own set of categories, enabling us to divide up everything that exists without overlap or leftovers, has considerable explanatory and prescriptive power. I examine two apparently unrelated arenas in which this notion is at work; namely, in the alleged discovery and subsequent physical "improvement" of the headwaters of the Mississippi River, and in the surgical alteration of intersex infants. In both cases, reconstruction is undertaken as a means of eliminating an ambiguity regarded (...) as "unnatural" —an "error" in nature that culture must correct. Elilminating ambiguity, in turn, enables accessibility. Fixing a person or a river firmly in a category allows us to have various kinds of access to them. For a river, the access is in part physical; it means being able to walk up to its headwaters without wearing hip waders. For a person, the access is both intellectual and social; only once I know what sex a person, is do I know how to treat them, and only then do I know whether they are an "appropriate" object of my erotic attention. (shrink)