Economics-Imperfect Science

Adam Smith's invisible hand to Keynesian play with Interest rates (liquidity preference). Friedman's dichotomy and still we had decade long Japan's deflation and then recent crisis.

Another root-cause identified for recent crisis..... Was it just one thing....

Why there is always so much of post-facto wisdom. Isn't true knowledge:
1) Can justify past
2) Could predict future(atleast a little)

Should I unlearn what they taught me in science.....Are all nobel-prizes in economics going in vain.......