I think recent events have shown all of us to suffer from mass delusion - i.e we have all believed in the financial system up to now. We have all agreed that a scrap of paper with a few pictures and numbers on it was intrinsically worth 5 or 10 or 20 or 50 or whatever dollars / euros etc., or that those digits and zeros in the computer represented real value.
As long as we all agreed to believe in it, there was no delusion and had I walked into a bank and burned up some stacks of paper pictures, I would have been jailed. And to some extent we all still believe in it. So are we delusional or not? If we all believe in it, it seems to work quite well. If we don't, it doesn't. But our mass belief produces tangible results, equally our disbelief produces tangible results.
Maybe the real question is not whether we are delusional (can we be 100% sure of ANYTHING in this world, or can we argue about it in circles forever) but which 'delusions' are worth bothering with.