The Inspector tells Mrs Birling that his work makes more of an impact on the young. We go on to see how Sheila and Eric are conscience-struck by their actions while Mr and Mrs Birling try to absolve themselves of blame. The young are flexible enough to change, the old aren't. This gives the audience hope that future generations will learn from the mistakes of the past.
Mr Birling is so full of his success and his future knighthood that he tries to brush away his responsibilities: the Inspector reminds him of his duties. Mr Birling has forgotten that without the labour of the working classes - whom he now tries to brush aside - his business would have gone nowhere. He has been dependent on people he now ignores.
Mrs Birling is proud to display her prejudice against Eva when she called herself 'Mrs Birling' in front of the committee. Mrs Birling's position, as an upper class influential lady in the town, put her in a position of power above Eva: she had no conscience about punishing what she saw as rudeness on the part of Eva by denying her money.
This is the main message of the play. The Inspector, the moral mouthpiece, speaks to the Birling family just before he leaves, hammering his point home by repeating We to re-iterate that all of us are involved. Priestley wants the audience to go away with this lesson.