These studies examined the role of ontological beliefs about category boundaries in early categorization. Study 1 found that preschool-age children (N = 48, aged 3-4 years old) have domain-specific beliefs about the meaning of category boundaries; children judged the boundaries of natural kind categories (animal species, human gender) as discrete and strict, but they judged the boundaries of other categories (artifact categories, human race) as more flexible. Study 2 demonstrated that these domain-specific ontological intuitions guide children's learning of new categories; children (N = 28, 3-year-olds) assumed that the boundaries of novel animal categories would be narrower and more strictly defined than novel artifact categories. These data demonstrate that abstract beliefs about the meaning of category boundaries shape early conceptual development.