There’s more to life than money, but societies can struggle to express it. When we talk about the state of nations and their citizens, we tend ultimately to boil it down to a few economic indicators. These can tell us a great deal, but they don’t quite tell us everything. The untouched stretch of ancient woodland, the arts in education, the close-knit community, the healthy childhood: there are plenty of valuable things that cannot always be weighed on this scale. And this is a problem, particularly for people who want to hang on to those valuable things. Here’s a rule of thumb: if you can’t measure how much something is worth, it becomes tricky to protect it. Those who do not want to see a bluebell wood destroyed or another humanities subject ditched can find it hard to argue their case against the cold logic of pounds and pence. What yardstick can they use that will be taken seriously? It’s not just activists for whom this is an issue. Governments have long flirted with the idea of an alternative to GDP, a way to capture what really matters to citizens. A “happiness index” was at one point a fashionable idea, but has proved too narrow and ambiguous. People often misunderstood what surveys were asking them – happiness can be hard to pin down. How do you weigh pleasure against contentment? What about meaning? But I wonder whether over the past few years a solution has presented itself, almost without us noticing. Little by little, a sturdier, better evidenced and more widely applicable unit of value has emerged in public life: that of mental health. We all use it already. Is it good for your mental health? This is the question by which increasingly large parts of our lives are evaluated. The language of psychology has spread so that it covers almost all human activity, from the quality of our workplaces, to the state of teenagers, to the way we spend our free time, to the manner in which we engage with our friends. If politicians are looking for a new way to measure what is important to us, they only have to open their ears. Take, for example, the simple hobby. People no longer merely go for a walk, take up birdwatching, adopt a pet or learn to bake cakes. Instead, they discover a form of self-care that conquers their anxiety, changes their life or saves their marriage. This is what we aspire to, now, in our free time – a radical overhaul of our mental health. Just look at the books hobbyists are writing these days. Sample: Hooked: How Crafting Saved My Life; Thirty-Thousand Steps: A Memoir of Sprinting Toward Life After Loss; Dinner for One: How Cooking in Paris Saved Me. The arts, too, are increasingly framed as aides to wellbeing. Why invest in a gallery pass? It’s good for your mental health. Why take up an instrument or learn to paint? It’s good for your mental health. Where once art was dedicated to the glory of God, now it is laid on the altar of mental wellbeing, which might well be the religion of our secular age. (The archbishop of Canterbury once complained that Christ the saviour had become Christ the counsellor.) Even fictional characters are these days evaluated in terms of their mental health. The writer Parul Sehgal has noted the rise of the trauma plot, which does not “direct our curiosity toward the future (will they or won’t they?) but back into the past (what happened to her?).” And we do this to real-life characters too. Channel 4’s recent The Rise and Fall of Boris Johnson is one in a long series of probes into the former prime minister that attempts to locate his future actions somewhere in his childhood. Tabloids used to fill their pages with claims about what gives you cancer (intermittent fasting, red wine, hot tea) or helps stave it off (intermittent fasting, red wine, hot tea). Now the story is wellbeing. What helps? What hinders? Now, it is easy to scoff at all this, and plenty do. Many are turned off by the spread of this newfangled language and what they see as the co-option of common sense. (What happened to just going for a swim or seeing your mates? Isn’t this “mental health” stuff all rather obvious?) Indeed, this scepticism was probably the basis for a nascent culture war launched last week by Rishi Sunak, who announced a plan to withdraw disability benefits from some people with mental health conditions, in a bid to tackle the “overmedicalising [of] the everyday challenges of life”. But these critics – and that solution – badly miss the mark. Learning more about our mental health, what helps and what doesn’t, is in fact rather useful. This knowledge was once the preserve of academics and those who could afford therapy; now anyone can get hold of it. This can only be a good thing. The evidence base may not be complete, but it is weighty and increasing all the time. The solution to “overmedicalisation” is surely not to ditch people in need but to continue to improve our understanding of mental health. And we shouldn’t presume, either, that complex large-scale societies always proceed in a healthy “commonsense” direction; it is new research into mental health that is guiding us back to “commonsense” practices – seeing your mates more, going for a swim outside – that we were in danger of leaving in the past. If we want an alternative unit of value by which to assess and guide a society, we could do worse than the mental health of its citizens. In a way, we’ve already adopted it. Despite Sunak’s posturing, mental health is a potent political tool. It was only recently that he launched a crackdown on mobile phones in schools on the basis that they harm children’s mental health. Mental health impact is the reason we now take loneliness seriously. It is the metric by which we increasingly judge workplaces; the government has previously urged employers to improve support. It is the justification for keeping hold of green spaces and, increasingly, for caring about the environment in the first place. We already measure ourselves by our mental health. Time to put that to use. Martha Gill is an Observer columnist