"When kings are building", says the German poet, "carters have work to do".Kronecker quoted this, in his letter to Cantor of September 1891, only to add,thinking of himself no doubt, that each mathematician has to be king and carter at the same time.
But carters need roads. Not seldom, in the history of our science, has it happened that a king opened up a new road into the promised land and that his successors, intent upon their own paths, allowed it to be overrun by brambles and become unfit for transit...