When we perceive something by seeing or hearing it, the sources of the impressions we hear or see are at a distance from our sensory organs. How is this distance bridged? Are there processes in the world of seemingly infinitely small things, in a microworld, that produce this connection?

Two ideas, which are essentially irreconcilable, lead to alternative explanations for bridging the space between source and appearance. Either the source of the appearance emits something that traverses space and evokes the appearance in a distant place, or some kind of material medium between source and appearance is changed locally into a different state from which it can only return to its original state if it transfers its changed state to the medium. In this way, the changed state propagates and spreads out in the medium without it necessitating for the medium itself to move from the source to the appearance.

In modern times, these two alternative notions found separate theoretical traditions. The first notion is closely connected with atomism. Matter is composed of the smallest particles in space that is otherwise empty. A radiating body emits small particles, which fly through the empty space and create as “particle rays” the perceived appearance in a distant place. The second notion leads to the model of a wave, which propagates in a medium, like a wave in the sea. Therefore, the space that is bridged cannot be entirely empty.