Although the Sun is frequently labeled as an “alternative” energy source, it has in fact produced almost all of man's energy throughout history. The world's energy economy now runs primarily on fossil fuels, a form of “solar capital” saved over geological time scales. Because this capital is apparently rather modest in amount and is not being renewed by nature at a rate comparable to our demands, we may eventually exhaust it. We will therefore be forced to turn either to the use of larger non-solar capital stocks such as primordial methane (if it exists), uranium-238 and deuterium, or to the use of regular “solar income” derived from the Sun's daily radiation.