Suffering from a profound depression in the year 1730, the young Samuel Johnson would walk from Lichfield to Birmingham and back in an attempt to raise his spirits; 90 years later, in 1820, the wit and cleric Sydney Smith wrote a thoughtful letter to his friend Lady Georgiana Morpeth listing his own attempts to fight off the condition. Now we’d call his letter a listicle – 20 things you can do to save yourself from depression. At number 14, he advised her to “Be as much as you can in the open air without fatigue.”

Other parts of his remedies are less Instagram-friendly, such as “Don’t expect too much from human life – a sorry business at the best.” But his advice on fresh air still stands, and it makes clear an important distinction between “fresh air” and “exercise”, even though the two used to fit closely together. Until quite recently, there was no way to separate the two. But the rise of gym culture has changed things.

Even insurance companies now demand exercise in exchange for cheaper insurance. But increasingly, doctors in the US and now in Scotland are prescribing fresh air, which is something rather different. The benefits of being among trees, or beside water, are strangely unrelated to the benefits to be had from sweating in a gym or pounding on urban pavements. For one thing, they tend to be much less sweaty. Smith’s advice to avoid fatigue is contrary to the spirit of gym culture. It is not cross fitness but equable content he’s after. Health may be what we need much more than physical perfection.

The natural world can be grotesquely uncomfortable and even dangerous. But it is also the place where our senses come alive. In the urban world, we live on sight and hearing alone, and spend much effort in shutting down even those senses. We are often plugged into headphones, and staring with grim desperation at the safe little rectangle of glowing screen where it seems that nothing really startling can ever intrude. Even before smartphones, much of the art of urban living lay in the skill of ignoring other human beings, whether they were beggars on the street or simply the strangers crammed into one another’s armpits on a commuter train.

In the open air, though, every sense expands. Touch and smell return from their long hibernation. Eyes are sharpened, and we start to hear the innumerable rustlings of lives that are not and should not be concerned with ours. There is no need to head for the great bare mountains for sublimity. Even an English parkland will remind us that we are guests on the earth. This is not only sane; it is a sense which we will have to cultivate if the earth is not to sweat us out in a dreadful fever. A prescription for walks in the open air should not be reserved for the lucky patients of a few Scottish GPs. It should be handed out to everyone, as preventive care and not just to those already overwhelmed with anxiety. And it is one intervention that the NHS can undoubtedly afford.