The Cambridge Introduction to Shakespeare's Tragedies

Macbeth clutches an imaginary dagger; Hamlet holds up Yorick's skull; Lear enters with Cordelia in his arms. Do these memorable and iconic moments have anything to tell us about the definition of Shakespearean tragedy? Is it in fact helpful to talk about 'Shakespearean tragedy' as a concept, or are there only Shakespearean tragedies? What kind of figure is the tragic hero? Is there always such a figure?