The key to understanding the problem is the marginal productivity of debt, a concept curiously missing from the vocabulary of mainstream economics. Keynesians take comfort in the fact that total debt as a percentage of total GDP is safely below 100 in the United States while it is 100 and perhaps even more in some other countries. However, the significant ratio to watch is additional debt to additional GDP, or the amount of GDP contributed by the creation of $1 in new debt. It is this ratio that determines the quality of debt. Indeed, the higher the ratio, the more successful entrepreneurs are in increasing productivity, which is the only valid justification for going into debt in the first place.