Medieval builders who died before their cathedrals were finished were lucky, because once a building's built, everything it could be is erased by what it is
There is a tendency in this country, rooted no doubt in some legacy of Puritanism, to value the practical at the expense of the imaginative. This is manifest in the daily elevation of "common sense" to a sort of virtuous paramountcy and in the dull espousal of the merely possible, which inhibits the urge to reach towards the impossible. It is manifest in our popular diet, in our embarrassed relegation of Swift and Carroll to the status of children's authors, in our suspicion of grand gestures.

The development of the built environment is constrained by an endemic inability or refusal to think big and to think joined-up – not because there is a lack of people who can think thus but because they are outnumbered by the janitors of a culture of negativity and the jobsworths of the risk-free. One might expect that pragmatism's primacy would result in, say, cities, transport systems, hospitals and stadiums that were the envy of the world. Some hope. We seem to regard the pursuit of the practical and useful as an endeavour that can only be jeopardised by the application of imagination. Imagination is tolerated as an add-on, or as a sort of cosmetic, when it should be the means of propulsion.

Anyone who has worked outside Britain (in, I'd venture, any field) must be aware of the prevailing receptivity to imaginative ad-hocism and the liberating attitude of can-do. Imagination is trammelled by systems that are adhered to, red tape if you like. All cultures suffer red tape. The more traditionally dirigiste a culture, the more adept it is, paradoxically, at circumventing its own red tape – it will have had a lot of practice. The British have grown cravenly obeisant of its rules and regulations. This may stem corruption, but those countries in which there is a moderate level of corruption are the ones that enjoy a successful infrastructure and, thence, a quality of life that is beyond our dreams so long as we are stuck at home. The 50% of the population that would emigrate if it could – a figure that should, but probably doesn't, shock our smugly inward-looking government – presumably wishes to do so because it believes that its dreams might be realised elsewhere: Britain puts the brake on its people's aspirations. The more astonishing statistic is that 50% of the population apparently doesn't mind.

The most satisfying buildings are not those that are built, but those that take more than a lifetime to complete

To reach the greener grass on the other side of the valley one necessarily travels hopefully: witness asylum seekers and pioneers. Nothing ever lives up to expectation. Nothing quite matches our hopes. It might be said that this is testimony to the power of pure imagination before it is applied in the exterior world, a process that will inevitably diminish it. Robert Louis Stevenson's dictum about travelling hopefully being a "better thing than to arrive" is a metaphor for the invariability of promise's deception. I was prompted to think of this broader application by the way that London is undergoing a more rapid physical change than at any time in my adult lifetime. It is a great cranescape.

Cranes are emblems of potentiality and promise. They presage tomorrow. Their profusion is heartening, a cause of optimism. And yet, we know that once the cranes have gone, once the engineer's steels are covered by the architect's glass, we will again be disappointed. For the real excitement, the true hope resides within the process. The end of the journey, the gleaming finite entity, cannot compete with the three-quarters-finished skeleton. It may be whole but, unlike a baby or a calf, it has nowhere else to go. Its development is over. And no matter how exhilarating it is, it robs us of the chance to imagine it. Its actuality obliterates our dreams of it.