• Some categories, like tall man or red, are graded;
  • Other categories, like bird, have clear boundaries;
  • Categories are not organized just in terms of simple taxonomic hierarchies.
  • The basic level depends upon perceived part-whole structure and corresponding knowledge about how the parts function relative to the whole.
  • Categories are organized into systems with contrasting elements.
  • Human categories are not objectively "in the world", external to human beings.