My father's reaction to the bank building at 43rd Street and Fifth Avenue in New York City was immediate and definite. “You won't catch me putting my money in there!” he declared, “Not in that glass box!” Of course, my father is a gentleman of the old school, a member of the generation to whom a good deal of modern architecture is upsetting, but I am convinced that his negative response was not so much to the architecture as to a violation of his concept of the nature of money. In his generation money was thought of as a real commodity (实物) that could be carried, or stolen. Consequently, to attract the custom of a sensible man, a bank had to have heavy walls, barred windows, and bronze doors, to affirm the fact, however untrue, that money would be safe inside. If a building's design made it appear impenetrable, the institution was necessarily reliable, and the meaning of the heavy wall as an architecture symbol dwelt in the prevailing attitude toward money. But the attitude toward money has, of course, changed. Excepting pocket money, cash of any kind is now rarely used money as a tangible commodity has largely been replaced by credit. A deficit (赤字) economy, accompanied by huge expansion, has led us to think of money as product of the creative imagination. The banker no longer offers us the safety: he offers us a service in which the most valuable element is the creativity for the invention of large numbers. It is in no way surprising, in view of this change in attitude, that we are witnessing the disappearance of the heavy-walled bank. Just as the older bank emphasized its strength, this bank by its architecture boasts of imaginative powers. From this point of view, it is hard to say where architecture ends and human assertion (说法) begins. The main idea of this passage is that______.