AFor most of human history, the typical settlement was small enough that nature lay only a short walk away. The modern city, by contrast, is something quite different: vast expanses of concrete, asphalt and glass in which the majority of the world's population now lives. In such an environment, the parks, gardens, tree-lined streets and pocket squares that planners refer to as urban green space have come to seem essential rather than ornamental. Once regarded chiefly as decorative features for the wealthier districts, they are now widely recognised as a quietly important kind of public infrastructure. The change in how planners and politicians regard them has been quite rapid, and its causes are worth examining.

BThe earliest large public parks in Western cities were a creation of the nineteenth century. As industrial cities grew at unprecedented speed, reformers became increasingly alarmed by the cramped, polluted conditions endured by ordinary workers. Parks were proposed as a partial answer: places where the urban poor might find fresh air, exercise and a measure of relief from crowded housing. Famous examples such as Central Park in New York, opened in stages from the 1850s, were planned with such social aims explicitly in mind. The new parks were also expected to provide a more orderly form of recreation than the alehouse or the street, and in this sense they were a kind of social experiment as much as an act of generosity.

CModern research has gone a long way toward confirming the intuitions of those early reformers. Studies in fields ranging from public health to psychology consistently find that people who live near green space report better mental and physical health than those who do not. A widely cited investigation in the Netherlands, for instance, looked at the records of hundreds of thousands of patients and found a clear association between the amount of greenery within a kilometre of a person's home and lower rates of conditions such as anxiety and depression. The effect was strongest among children and people on lower incomes — precisely the groups for whom such spaces had originally been intended.

DThe benefits of urban greenery extend beyond the human inhabitants. Trees and other vegetation cool the air through the evaporation of water from their leaves, reducing the so-called "heat island" effect that can make city centres several degrees warmer than the surrounding countryside on hot summer days. They intercept rainwater that would otherwise overwhelm drains and cause flooding, capture particles of pollution from the air, and provide habitat for an unexpectedly rich variety of wildlife. Some city parks now host more species of bird and insect than the intensively farmed land beyond the city limits, a striking reversal of the usual relationship between town and country. Even relatively modest amounts of vegetation, well distributed, can make a measurable difference.

EYet not all green spaces deliver these benefits to the same extent, and access is far from evenly distributed. In many cities, the leafiest neighbourhoods are also the wealthiest, while poorer districts may have little more than narrow strips of grass beside busy roads. Researchers refer to this disparity as "green inequality." There is also a paradox sometimes called "green gentrification": when a park is improved or a new one created in a deprived area, the surrounding neighbourhood often becomes more attractive to wealthier buyers, pushing up property prices and pushing out the very residents who were meant to benefit. Planners are increasingly aware of this risk, but solutions are difficult.

FFaced with limited land in established cities, designers have grown inventive. Disused railway lines have been converted into elevated parks, of which New York's High Line is the best known. Rooftops are being planted with gardens that insulate the buildings beneath them while creating new public space, and there is much talk of "living walls" covered in plants. Some cities have set targets to ensure that every resident lives within a short walk of a substantial green space — a deliberate echo of the social ambitions of the nineteenth century, now framed in the language of environmental policy.

GIt would be a mistake to imagine that green space alone can solve the problems of modern cities, or that simply planting more trees will compensate for poor housing, traffic or pollution. What the evidence does suggest, however, is that the way a city is designed shapes the lives of its inhabitants in countless small and cumulative ways, and that the presence or absence of accessible greenery is among the most important of these factors. The park, once a luxury imported from the countryside, has become something close to a necessity — and the question for many cities is no longer whether to invest in green space, but how to share its benefits more fairly. That question will become more pressing as cities continue to grow and as climate change places further stress on urban life.

AWe tend to think of memory as a kind of recording device, faithfully storing events for us to play back later. The events of childhood, an old friend's face, the conversation we had this morning — these feel less like reconstructions than like accurate replays of something that genuinely happened. Yet decades of psychological research now indicate that this intuition is profoundly misleading. Memory, it turns out, is not so much a recording as a reconstruction: an active process in which the brain assembles, and frequently revises, the past. The implications of this discovery extend from the privacy of personal recollection to the public business of the courtroom and the writing of history itself. Few findings in modern psychology have proved so unsettling, or so widely applicable.

BModern thinking about memory owes a great deal to the work of the psychologist Sir Frederic Bartlett, working in Britain in the early twentieth century. Bartlett asked his volunteers to read an unfamiliar story — typically a folk tale from a foreign culture — and then retell it from memory after various intervals of time. He found that they did not simply reproduce the story; they altered it, often without realising. Unfamiliar details were quietly replaced with more familiar equivalents, and the narrative was reshaped to fit the cultural assumptions of his English participants. From these experiments Bartlett concluded that recollection is not passive playback but active reconstruction, shaped by the storehouse of expectations and knowledge that each of us brings to it.

CThe most striking modern evidence for memory's fragility comes from research into what is known as the misinformation effect. Professor Elena Garcia has spent decades showing how easily a witness's recollection of an event can be altered by misleading information encountered afterwards. In a typical study, participants watch a brief film of a car accident and are later asked questions about it, some of which contain false details. When questioned again later still, many participants "remember" those false details as part of the original scene. Their confidence in these mistaken memories is often high, and they cannot tell the difference between what they witnessed and what they were merely told.

DEven more unsettling than the misremembering of details is the possibility of an entire false memory: a confident recollection of an event that never took place at all. In a celebrated series of experiments, participants were persuaded by family members' testimony that they had once, as a child, been lost in a large shopping centre. Despite the event being a fabrication, a substantial minority went on to "remember" it in considerable detail, sometimes adding elements of their own. The psychologist Dr Marcus Lim, who has reviewed many such studies, argues that the brain shows little reluctance to construct memories of events that never happened, provided the suggestion is repeated, plausible and comes from a source the rememberer trusts. Such findings have transformed how psychologists think about the boundary between imagination and recollection.

EWhy should memory work in this strange and unreliable way? The cognitive scientist Professor Anna Reuter has proposed that what looks like a flaw is in fact a consequence of memory's real purpose. Memory, she argues, did not evolve to provide an exact record of the past, but to help us anticipate and respond to the future. From this point of view, it makes excellent sense for the brain to update old memories in the light of new information, to blend related experiences together, and to throw away most details as soon as they have served their purpose. A perfect, video-like archive of every moment lived would be cognitively expensive and largely useless; a flexible, gist-based system is faster and more practical.

FThe practical consequences of this research have been felt most sharply in the legal system. For most of its history, the testimony of eyewitnesses was treated as nearly the strongest form of evidence available, and convictions were routinely obtained on the basis of confident identifications in court. Yet when criminal cases have been reopened using DNA evidence, mistaken eyewitness testimony has turned out to be a major factor in wrongful convictions — in some studies, the leading cause. The barrister Dr Olufemi Adesanya, who has written widely on the subject, argues that this should not in any way be taken to discredit witnesses, who are usually honest and doing their best; rather, it shows that even sincere, confident memories can be mistaken, and that legal procedures need to be designed with this fact in mind.

GWhere does all this leave us? Some commentators have responded with dismay, suggesting that if memory is so unreliable then our very sense of self, built from what we remember of our own lives, must be on shaky foundations. The historian Dr Beatrice Lindqvist, who has examined how individuals reconstruct their own pasts, takes a more measured view. The fact that memories are partly constructed does not make them worthless, she argues; the autobiographical story each of us carries is a meaningful interpretation of a real life, even if some of its details are subtly wrong. What the science of memory teaches, on this account, is not to distrust our memories altogether, but to hold them with a certain humility — and to recognise that the past, like the future, is partly a creation of the mind that contemplates it. In this sense, learning how memory really works may, paradoxically, leave us on firmer ground than the comforting illusion it replaces.