 In 1979, John Perry and David Lewis produced examples purporting to show that this simple way of theorizing about belief was inadequate, because the family of doxastic states at its disposal—states of believing that P for traditional propositions P—couldn’t explain some bits of behavior that it’s important to be able to explain in doxastic terms, couldn’t capture certain sorts of ignorance and doxastic change, and/or couldn’t properly categorize believers so as to capture important doxastic similarities and differences between them.