the inspector is a foil to Mr Birling (opposite)
The inspector is the direct opposite of Mr Birling & is used to teach the audience about his character.
The inspector crystallises his socialist beliefs within the phrase "we are members of one body. We are responsible for each other"
his final speech, sees a strong socialist message from Priestley as the inspector is his mouthpiece.
The anaphora of the collective pronoun "we" directly contrasts Mr Birling's egocentric views.
difference of socialism/capitalism Mr B uses "I", concept of collectivism is dismissed by Mr B in act 1