S'Kra Archmage from Muspar'i who, through painstaking mundane and arcane research, as well as rigorous tests and experiments, was able to ascertain the existence of a sixth element, what we identify as Aether. The evidence, and the work Hh'reshh engaged in in order to obtain his evidence, is laid out in his exemplary tome, "The Principles of Elemental Magic." His findings have been accepted as magic theory canon.
Source: Treatise on the Elements, Volume 1
He also theorized that the power of Elemental Magic was drawn not from our Plane, but from others, the Elemental Planes. He stated that, if such Planes did exist, then they would be the source of elemental power and, just as in the Plane of Abiding, there would be beings living there. Those beings are called elementals.
Source: Tale of Paeldryth
Note: Although the exact dates of his life are undocumented, Muspar'i was completed in 225 BL, so he can have been born no earlier than that. This appears to contradict The History of the Warrior Mage Guild, Volume 1. Given the nature of the statements in Treatise on the Elements about the discovery of Electricity, which was previously known but that the Gealeranendae College of Magical History verified the existence of the Plane, we can probably safely assume that his discovery of Aether was also more of an empirical verification of its existence.