To understand how language works, we look, fi rst, to the cooperative functions that various language forms perform, understanding these on a biological model as what they accomplish for both speaker and hearer that keeps them in circulation. Next we look at language mechanics, at how language forms perform their functions. For many (but not all) language forms there are conditions in the world that would be necessary to support their functions and that vary systematically with certain variations in the forms themselves. These are truth or satisfaction conditions. Last we describe the psychological mechanisms that are involved in implementing the functions of various language forms. For extensional terms these involve methods of identifi cation of incoming information about the extension of the term, methods that are usually multiple and various for each person and that may also vary widely from person to person.