Imagine for a moment that the entire human race, from the time of our emergence to the present day, is one person, now 80 years old.
Now, imagine that you are that person.
For the vast majority of your life, up to about three days ago you have lived in the midst of nature, in the company of animals, trees, birds and vegetation.
You roamed with a close-knit band of maybe 20 or 40 others, being unused to seeing crowds of people in one place.
You hunted and searched for food, always walking, sometimes hundreds of miles. You slept in trees, caves, temporary shelters or under the stars.
Nearly every day of your long life you have heard the cries of birds and other animals, the rush of water, the wind in the leaves, and have known intimately what these signs mean.
Your fears have been of hunger, of being killed by beasts, and of the mysteries in the world around you.
Still, there have been important developments.
You have acquired speech, discovered fire, and developed increasingly more sophisticated tools.
You have explored the earth, learnt how to survive in many different environments, and stored your knowledge in memory and stories.
And so, just about four days ago you discovered how to grow your own food from seeds.
You settled down with a few other bands of fellow humans, grew many different crops, kept animals that were no longer wild, and built permanent structures. By the next day your settlement had expanded to several thousand people.
Some of these didn't work on the land; they began to make things, to rule, and to trade.
This morning, you invented writing and reading.
A few hours ago you developed powerful new tools with which you could make many more things. Your city grew to ten times its size.
You no longer saw woods and fields, only streets, chimneys belching smoke, and blackened walls.
Your children forgot how to live with nature.
In the last few minutes your city has become enormous and you have been bombarded with many more new technologies and unfamiliar effects of your actions - chemicals, pollution, noise.
You eat only highly processed food, scarcely knowing where it came from.
You do not know your neighbours.
You hardly walk any more, where you used to walk 10-20 miles a day.
You do not know wild places.
The only animals you see are pets, or in cages.
You have, often without realising it, destroyed countless species.
Machines support you, shield you from the world and surround or even occupy your body.
In fact, the human species is 3 million years old.
Life on earth took 4 billion years to evolve. Our own evolution is part of that long process.
And we carry within ourselves genetic and structural remnants of all stages of that development.
Our affinity with life on this planet has its basis in our very cells and building blocks, the stuff from which we are made.
The natural world is hard wired into us.
Surrounded by our built environments we tend to forget this. But our bodies and our unconscious minds cannot.
Biophilia, a theory propounded by Edward Wilson, gives scientific credence to what we all feel must be true - that we need nature for our physical and mental health.
Our very sinews and nerves demand it, and when denied it, suffer.
This is why human beings "have an innate tendency to focus on life and lifelike processes" as he says.
Study after study has confirmed this view. Some of the results of these are less surprising than others. For example:
* If patients in hospitals or prisoners in their cells are given a view through their window of nature, rather than a view of buildings, then they experience reduced medical problems.
* If people living in urban communities get together to create a garden or park space, they often discover that this process creates a social 'glue', binding them together. Coincidentally, crime and other social problems can be reduced.
* all other things being equal, houses situated close to a park have a higher resale value than those which are not.
* people who are about to sit an exam or perform a stressful task do it better if they have spent a few minutes in a calm, greenspace environment just beforehand.
* Tests on North American and European adults have shown that they prefer to look at natural landscape scenes rather than urban ones.
* Increasing access by members of minority non-indigenous individuals in the UK to nature reduces their feelings of alienation and can help them feel more connected, both to their country of origin and to their current country of habitation.
* victims of torture undergoing recovery programmes have been found to have a faster rehabilitation rate when they are allowed access to gardens, particularly if they can cultivate their own plots.
Research such as this serves to establish that there is a rational scientific basis for what is generally dismissed as emotionalism. This makes it more likely that natural elements can be included in planning initiatives, whether for a new hospital or an urban renewal programme.
But, given the kind of world we live in, it's even more likely that this will happen if it can be established that there is also an economic benefit. Usually this can be found in reduced crime and healthcare bills.