This is an excellent introduction to the philosophical problem of universals: what is it for two things to be of the same type? Armstrong discusses the pros and cons of six theories: 1) natural classes as primitives; 2) resemblance nominalism; 3) universals; 4) natural classes of tropes; 5) resemblance classes of tropes; and 6) tropes plus universals. He also discusses substance-attribute and bundle theories of universals and tropes.
Armstrong’s own position is that the world consists of states of affairs, which are made up of substances with attributes, that is, particulars that instantiate universals, which have differing degress of resemblance. Although he believes that universals are preferable to tropes, Armstrong sees resemblance classes of tropes as a strong competitor to universals. He also suggests that the differences between universals and tropes may be merely terminological. (3/9/10)
An Ingenious Device for Avoiding Thought 