alan greenspan. anthony? >> reporter: scott, alan greenspan says jeopardizing trust in u.s. credit and effectively the u.s. dollar is very dangerous. at his home in washington today, the economist who ran the federal reserve for 18 years told me hitting the debt ceiling will force the president and the treasury to decide who to pay and who to delay. >> you may very well choose to pay interest to the debt and to whom does it go? china. and who don't we pay? social security. that is a political system which will not hold together. i think they have to resolve it but the shock to the system, i think is -- not necessarily permanent but really debilitating. >> reporter: so we're playing with fire? >> indeed, we are. and then there's the psychological impact. >> reporter: you don't want to test it? >> i'd just assume not. >> reporter: nobody really know,