"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. [..] “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. [..] 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.“