Increasingly incoherent speeches, a highly volatile temperament and brazen narcissism. The president’s behaviour is much easier to understand, writes Alan Rusbridger , if we all come to terms with one thing T he American constitution is creaking at the seams. The founding fathers got many things right, but ultimately their imagination failed them. They could not, in their worst nightmares, conceive of a president who would be simultaneously all-powerful and mentally unwell. So here we are