Category Induction and Representation: Online text of a chapter by Stevan Harnad in `Categorical Perception: The Groundwork of Cognition'.
Natural Kinds and Projectible Predicates: In this article Axel Mueller applies some reflections upon Goodman's paradox to the problem of natural kinds.
Properties: Entry in the Stanford Encyclopaedia of Philosophy by Chris Swoyer. Principally concerned with existence and identity conditions.