No one can see it, hear it, smell it or touch it, and yet people seem to have sensed its existence for nearly as long as there have been written languages.

Twenty-three centuries ago Aristotle inferred its existence by proclaiming that nature abhors a vacuum. In ancient Rome, the Stoics believed in a universal medium they called the ''pneuma.'' In medieval times, thinkers called it the ''plenum,'' and later, it went under the name ''ether.'' This ethereal stuff, in somewhat modified form, survived even the investigations of modern science, even though it seemed for many years that Einstein had consigned ether to the trash heap of obsolete notions. In its latest incarnation, ether has become a universal web of ''quantum fields.''

This stuff is hard to define, but it is what you have when there is nothing else. Theorists spend a lot of effort thinking about it.

In the 19th century, with the realization that light travels in waves, many scientists believed that there had to be some universal medium, the equivalent of an ocean of water, through which light waves could travel even in the vacuum of space. This implied that something -- the ether -- pervaded every nook and cranny of the universe, including the empty space within atoms and between galaxies.