I guess it depends on how much of an aberration the fossil-fuel era ends up being. For thousands of years economies (well, measuring real incomes, not "economies") didn't really grow:
For economies to continue to grow as they have been for the past few generations, we'll probably need some substantial technological breakthroughs, either in the energy sector, or by making the things we use much more efficient in their use of energy. Otherwise, I'd suspect a convergence on historical norms. So I guess the leap of faith is believing either a) we can create an equivalent to one of the greatest stores of energy in the known universe (fossil fuels), or b) restructure our economy to do without them but continue to grow.