METAPHORS matter. The right one can suggest new lines of inquiry and non-obvious solutions to pressing problems. But pick the wrong one and you risk being misled by false analogies and blinded to better approaches. The metaphor of “stress” for mental or emotional strain or tension has shaped thinking about mental health since it was coined in the 1930s (see article in this week’s issue). Borrowed from physics, it suggests that people can withstand adverse or demanding circumstances up to a certain point, after which they will break. Yet it is wrong. New studies suggest that the mind is more like a muscle than an iron bar—weakened, not protected, by being saved from significant challenges. To grow stronger it needs to tackle hard tasks in fruitful ways—and to be allowed to recover afterwards. For workers and firms alike, the lesson is that difficult tasks encourage growth, recovery time should be built into work and personal time should not swallowed up by social media and e-mail.

Another physics metaphor, that of “pressure”, suggests that people put under too much of it will explode. This may be one reason why legal systems have historically been rather forgiving of men who go on rampages after too much wifely nagging or losing their jobs. The notion that “the body is a temple” misleads slimmers and health freaks into pursuing purity and eschewing contamination when choosing foods. That can cause malnutrition and eating disorders—and supports a vast, quack-ridden diet industry. “Calories in, calories out” is more than a banal restatement of the Law of Conservation of Energy: it is a metaphor casting the metabolism as akin to a current account. Weight gain is then simply a matter of depositing more than you withdraw. But that ignores the role of hormones and appetite; differences in the way different foods are metabolised and the way the body reacts to prolonged deprivation by hoarding fat and slowing down. No wonder diets rarely work.

Biologists probably know what they mean when they describe DNA as “the software of life”. But for laypeople it omits that the environment influences the way in which DNA’s instructions are followed, and leaves untouched the old, sterile dichotomy between nurture and nature. The brain has been, in turn, likened to clay infused with spirit, a hydraulic device driven by liquid “humours”, a clockwork engine—and now a computer. Belief in humours led to such disastrous treatments as bloodletting and purging; the computer analogy is also misleading. Brains neither store memories nor process information in any form similar to a digital computer. When they go wrong it is not because of faulty coding.

The open ocean has long been a metaphor for eternity and boundless freedom. By encouraging humans to think it can be raided with impunity and used as a bottomless dumping ground, the metaphor has led to overfishing and pollution. When migrants are perceived as a “flood”, not only are they dehumanised but their movements are also mis-described. Migrants come and go; many will eventually return home; and even as some are entering a country others are leaving. Call them a flood and you will concentrate on building a dyke and plugging the leaks; understand them as ebb and flow and your mind will naturally turn to what attracts and repels them.

If you think of inspiration as arriving in lightbulb moments you may sit around waiting for it; if you see it, instead, as the result of a seed planted in fertile soil you will seek to improve that soil and stock it well. If you think talent is a treasure possessed from birth, you will believe too easily that if you cannot do something now, you never will. And if, for you, a deal is simply a matter of dividing the pie you will neglect to seek ways to make the pie bigger—or, indeed, to ask yourself how pies get baked in the first place.

Perhaps the most ubiquitous metaphor is that of war. Sometimes it is helpful: the “war on AIDS” could actually be won. But others cannot, and then the metaphor may mislead. Wars are waged by coherent enemy forces and end, if not in victory, at least when both sides are on their knees. The “war on cancer” has encouraged doctors and patients to view bodies as battlefields. In the “war on drugs”, addicts who need health-care are recast as enemy combatants. And the “war on terrorism” has fostered the delusion that, with a final push on a battlefield somewhere, it can be won.