When the ubiquitous quantum, acting as an active principle, generates meteons in the System of the World, the Absolute Certainty Principle (ACP) regulates the characteristics of their motion. This newly uncovered law of Nature suggests that the cosmos is filled with an "aether", as Newton and others-even Einstein!-called it in their days, and explains quite simply why we stand erect vertically on the surface of the Earth and why the universe is in expansion.