Alchemy, natural philosophy, and early modern physics proposed the existence of a medium of the æther (also spelled ether, from the Greek word (αἰθήρ) aether, meaning "upper air" or "pure, fresh air"), a space-filling substance or field, thought to be necessary as a transmission medium. The assorted aether theories embody the various conceptions of this "medium" and "substance". This early modern aether has little in common with the aether of classical elements from which the name was borrowed. The Æther is considered the global reference frame for the Universe and thus velocities are all absolute relative to its rest frame. Therefore, in this view, any physical consequences of those velocities are considered as having an absolute, ie real effects. 275 pages.