Why are some countries rich and some countries poor? Is it access to natural resources? Is it tax policy? A motivated work force? These are important, but not determinative. The answer is deceptively simple - it's what's in our heads: knowledge. Thus, the surest way to promote economic growth is to cultivate an environment that encourages the spread of knowledge. Such an environment requires freedom, which is why the freest societies are the most prosperous. In five minutes, economist George Gilder explains why.