essay on Hamlet , (7) he writes:

          The only way of expressing emotion in the form of 
     art is by finding an ‘objective correlative’; in other words, 
     a set of objects, a situation, a chain of events which shall 
     be the formula of that particular emotion; such that 
     when the external facts, which must terminate in sensory 
     experience, are given, the emotion is immediately evoked. 
     If you examine any of Shakespeare’s more successful 
     tragedies, you will find this exact equivalence; you will find 
     that the state of mind of Lady Macbeth walking in her 
     sleep has been communicated to you by a skillful 
     accumulation of imagined sensory impressions; the words 
     of Macbeth on hearing of his wife’s death strike us as if, 
     given the sequence of events, these words were 
     automatically released by the last event in the series.