Remember the Tory leadership contest? One of its most interesting and least discussed aspects was Liam Fox’s decision to launch his leadership bid from a small mental health charity in north London. Unusually, Fox made his claim to be leader of the Opposition by talking about mental health. There are many reasons why the centre left would reject his argument, which was steeped in Thatcherite suspicion of government. Nonetheless, it was noteworthy for being one of the rare occasions when mental health was at the forefront of a very public and unmistakably political speech.

As a rule, political speeches on the nation’s state of mind are unusual. Quite simply, mental health is not an issue that elections are fought over or won on. Mental health, however, is fundamental to everyone’s quality of life: mental health problems are very common and are a personal disaster for those who experience the most severe illnesses. Yet the theme of mental wellbeing has rarely featured in past discussions of the good society or accounts of social justice.

But there are signs that this is changing. The health secretary, Patricia Hewitt, said at a conference fringe meeting last year that our account of social justice needs to be updated with a broader and deeper understanding of wellbeing. Her comments reflect a growing interest in wellbeing that has been given fresh momentum by the work of the economist and Labour peer, Richard Layard.

Layard revisits the old wives’ tale, ‘money can’t buy you happiness,’ and finds it to be true. In short, western societies have grown richer, but have not grown happier. Prosperity has been accompanied by rising levels of unhappiness. There are more people with depression, alcoholism and drug problems. Teenagers are more likely to experience behavioural problems and emotional problems than 30 years ago. People over 65 are at greater risk of isolation and loneliness. In one survey by the Local Government Association, around a million older people said they felt lonely and acutely isolated in their own homes.

Loneliness, depression, addiction – there are many different problems that indicate poor mental health. But it is also important to be clear what good mental health looks like. Mental health is more than the absence of a problem; it is the emotional resilience that enables us to enjoy life, cope with setbacks and adversity and respect and relate to other people. Mental health is a social good that underpins quality of life. As such, we need to protect it and promote it, just as society seeks to protect clean air and green spaces. Moving towards this view of mental health would be a significant change from our current approach, where mental health issues are bundled into a narrow policy box ‘belonging’ to psychiatrists and ‘other people’.

Many people have heard the statistic that, at any one time, one-in-six people have some kind of mental health problem. But there is less of a clear sense of the scale or impact of lost opportunities and personal and family unhappiness. The cost of mental health problems, both financial and human, is substantial. Around 10 per cent of five-to-15 year-olds have clinically significant mental health problems. Levels of self-harm among teenagers in the UK have doubled in the past decade and are among the highest in Europe. There are almost one million adults claiming incapacity benefit due to mental ill health. For many people, this premature exit from the labour market erodes confidence and self-belief, which is one factor that hinders people from returning to work even when they would like to.

Mental illness is written in invisible ink on many of our most pressing social problems, such as worklessness and poverty. The links between mental ill health and poverty should add a new dimension to thinking on how to achieve social justice. As Patricia Hewitt suggested, there is a connection with social wellbeing and social justice. While poor mental health is not the preserve of any one group, it disproportionately affects low-income groups. Poverty and mental illness reinforce each other in a vicious cycle. People living in poverty are twice as likely to get depression as people on high incomes. Likewise, long-term mental health problems can make people poor and keep them poor. People with long-term mental illnesses are one of the most excluded groups in society. A compendium of statistics show that the life experience of this group is more likely to be characterised by unemployment, low skills, bad housing, poor health and fewer friends than average.

Prioritising people’s mental wellbeing pushes some policy areas up the priority list. As Tania Burchardt argues in the ippr book Social Justice, an understanding of wellbeing directs attention to giving people a chance to recover from setbacks. Unforeseen problems, failure, or just plain bad luck all get in the way of people making the most of their opportunities. Burchardt draws the analogy that, ‘if a policy referee blows the whistle at the start of the game and then leaves the pitch, fair play is not likely to ensue, the injured will go untended and morale will quickly slump.’ The government needs to pay increased attention to policies directed to maintaining people’s hope and resilience, such as adult education, offender rehabilitation and the treatment of mental health problems, which are focused on rebuilding aspirations.

To create a fair and thriving society we need to do more than react to the consequences of mental illnesses. It is also necessary to pay attention to the business of promoting good mental health. Mental health promotion reinforces the importance of early years programmes, such as Sure Start, and resources for high-quality childcare. The ambitious goals of ‘an equal start’ and ‘equality of opportunity’ can only be realised when children have self-esteem, emotional resilience and the ability to relate to other people, as well as economic resources and educational opportunities.

Thinking about mental health also puts the spotlight firmly on quality of life and social cohesion. Good mental health underpins and creates social bonds, mutual trust and respect. Better mental health is also associated with strong, tightly knit communities. Communities that share good levels of mental health also tend to have high levels of ‘social capital’ or ‘connectedness’.

Critics might complain that this is simply summarising the domestic agenda under a new heading. But a perspective on mental health adds a new and important dimension to our understanding of social problems and social justice. First, this perspective acknowledges the importance of mental health problems and how they can reinforce existing injustices. Second, understanding mental health can be integral to achieving other policy objectives, from child development and reducing worklessness to fostering mutual respect.

When Liam Fox was running for the leadership, one of his colleagues told him he would not vote for him if he kept talking about mental health because people ‘don’t want to hear about that sort of thing’. People who care about social justice should not make the same mistake.