Birling has a selfish attitude towards life, and only cares for himself and his family, basically forgetting everybody else; in fact, this is exactly what he tells a speech on at the celebration of Sheila and Gerald's engagement, " a man has to look after himself- and his family too, of course- this gives the impression of selfishness and also greed
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The inspector is a tool used by Priestley, used to literally question the position of the characters and what they represent. He is the catalyst for the unfolding of the plot. He is central to getting the characters to examine themselves and their actions and indirectly getting the audience to look at themselves make them think what they have done wrong and whether they can correct it. He moves from character to character with relative freedom as a police inspector. .
Mrs. Birling can be considered along with Birling, she represents the same echelon of society and all its attendant values. Also the fact that she, along with her husband learns nothing suggests to us that Priestley thinks the members of this class are blinkered and that their days may be numbered. .
Although Sheila has been brought up in the same way as her family, in the same upper class atmosphere, she has a totally different attitude to the situation than the rest of them. She realised she had done wrong in getting Eva Smith sacked from her job, for reasons such as she was having a bad day, and she was jealous of Eva's beauty. Sheila had done wrong, and given time and the current circumstances, she accepted this and had pity on her parents for not thinking in the same way. The fact that Sheila undergoes this journey of spiritual regeneration, from ignorance to understanding makes her a more rounded character than the inspector or Birling, she is less one-dimensional than the pair of them however she is only used in the play as a literally device. .
Eric can be considered in the same light as Sheila, together they represent the younger generation, well, at least the thinking and intellectual of the younger generations.