The restriction that probabilities be non-negative means that events can only appear in quadrants I and IV of the graph.
Why do we restrict the probabilities to be non-negative? Is there anything in particular that keeps us from pulling an Aaronson and generalizing probability to include negative and complex components, even absent a clear motivator like QM?
Why do we restrict the probabilities to be non-negative? Is there anything in particular that keeps us from pulling an Aaronson and generalizing probability to include negative and complex components, even absent a clear motivator like QM?