Category Induction and Representation - Online text of a chapter by Stevan Harnad in `Categorical Perception: The Groundwork of Cognition'. - http://cogsci.soton.ac.uk/~harnad/Papers/Harnad/harnad87.categorization.html
Natural Kinds and Projectible Predicates - In this article Axel Mueller applies some reflections upon Goodman's paradox to the problem of natural kinds. - http://sowi.iwp.uni-linz.ac.at/Sorites/Natural_Kinds.html
Properties - Entry in the Stanford Encyclopaedia of Philosophy by Chris Swoyer. Principally concerned with existence and identity conditions. - http://plato.stanford.edu/entries/properties/