The theme of class is key to understanding 'An Inspector Calls'. The underlying tension between the different social classes in the UK is presented by the playwright J.B. Priestley to explore ideas of social responsibility and gender in 20th century Britain. 

J.B. Priestley was a socialist who believed  that wealth should be redistributed to help poorer people live healthier, happier lives. Key to this was the argument that extreme personal wealth should not be allowed to be concentrated in the possession of individuals. 

Furthermore, Priestley argued that the wealthiest in society had a responsibility to help the less fortunate to reduce the suffering caused by poverty. This is shown by the Inspector making constant reference to Mr. Birling's duty to his employees and to Mrs. Birling's duty to the people who request help from her charity committee. The play also highlights the class tension between wealthy people (who paid taxes) and their expectations of the police (who are paid through taxes) and the belief that wealth and social status would enable them to sidestep any legal problems.   

J.B. Priestley uses the character of Inspector Goole as a mouthpiece to convey his socialist beliefs. In its most basic form, socialism is the idea that all members of society are interconnected; the success and wellbeing of the individual depends on the success and wellbeing of society as a whole. 

In reality, this approach emphasises the responsibility of the individual to others in society. This can include, but is not exclusive to, respecting others opinions, sympathising with different experiences, sharing wealth, time and hospitality with those who have less, limiting personal disagreements and assisting charities. 

Priestley compares social responsibility to a moral or religious duty, "we are all members of one body" paraphrases Christian religious observances.  

As a socialist, Priestley was keen to highlight the importance of changing attitudes to society and was particularly keen to demonstrate the important role that young people would play in changing society after the end of the Second World War. 

The play features an increasing tension between younger and older characters as Sheila and Eric begin to pull away from the influence of their parents (and by proxy, older, traditional views of society). The fate of Eva Smith (a younger woman from the working class) contrasts with the opportunities and behaviour of Sheila Birling (a younger woman from the upper class), while Priestley emphasises the choice that young people have in changing the future, when the Inspector notes to Mrs. Birling, when she remarks that he has "made quite an impression" that "we often do on the young ones. They're more impressionable." There is also the imagery associated with the death of Eva Smith and her unborn child, conveying the fragility of optimism and youth. 

Throughout the play, Priestley demonstrates the conflict brought about by an imbalanced society and how if affects different groups of people. This conflict is clearly shown in the role of gender. 

Eva Smith (a young woman) is driven to suicide by the treatment of people whom she turned to for help and support. However, despite being mistreated by both Mr. Birling, Eric and Gerald Croft, Eva is also harmed by the actions and attitudes of other women in the play (namely, Mrs. Birling and Sheila). 

Eva Smith is shown as a vulnerable character, made all the more vulnerable by her gender. As a working class woman in a patriarchal society, she is portrayed as powerless, isolated and alone. The imagery of powerful men who cannot be trusted to use their influence and power to care for others, notably women, is an especially provocative element of the play. The role of women in providing a counter balance to the thoughtless actions of male characters is steadily replaced by a mounting sense that women are as much to blame for social problems as men. 

The photograph represents Inspector Goole's power; he uses it to direct the line of inquiry and the order of who he questions. It also reinforces the idea of the male gaze; the photo is an object, as is Eva. All characters are given no option but to look (“gaze”) upon the image. 

Or, it could be argued that the photo is a symbol of the characters’ guilt; by being compelled to look at the photo, they must face the consequences of their actions. 

However, it could be interpreted that Eva Smith still has power over each of the characters, by being present after death. Through her, they are forced to look at their own misdeeds head-on. 

By never seeing the photograph, the audience is also allowed to build up their own image of Eva Smith, making the message of the play far more personal and emotive. 

Priestley uses the the theme of morality to encourage the audience to change their understanding of responsibility, to show that acting to help others is the correct (or moral) way to behave. 

Other than Eric (who stole money from his father's business and had implied non-consensual sex with Eva Smith), none of the characters have actually committed a crime. In this way, Priestley uses the play to show the distinction between moral (or correct) behaviour and immoral (or incorrect) behaviour. 

The way that characters attempt to justify their immoral behaviour to Inspector Goole serves to highlight not only the implications of their actions on others, but also the close-mindedness of their lives and behaviour.

The Inspector's closing speech makes it clear that if people are not willing to learn a lesson about their immoral behaviour through the play, then "they will be taught it in fire and blood and anguish", a reference to the two world wars which had happened in 1945 (when the play was written) but had not yet occurred when the play was set in 1912.