"It is a necessity in science, but intangible, undemonstrated, unknown, and wholly theoretical. It is accepted as an existing fluid by scientists, because human theory can not conceive of a substance capable of, or explain how a substance can be capable of affecting a separate body unless there is an intermediate medium to convey force impressions. Hence to material substances Aristotle added (or at least made conspicuous) a speculative ether that, he assumed, pervades all space, and all material bodies as well, in order to account for the passage of heat and light to and from the sun, stars, and planets." "Explain further," I requested. "To conceive of such an entity we must imagine a material that is more evanescent than any known gas, even in its most diffused condition. It must combine the solidity of the most perfect conductor of heat (exceeding any known body in this respect to an infinite degree), with the transparency of an absolute vacuum. It must neither create friction by contact with any substance, nor possess attraction for matter; must neither possess weight (and yet carry the force that produces weight), nor respond to the influence of any chemical agent, or exhibit itself to any optical instrument. It must be invisible, and yet carry the force that produces the sensation of sight. It must be of such a nature that it can not, according to our philosophy, affect the corpuscles of earthly substances while permeating them without contact or friction, and yet, as a scientific incongruity, it must act so readily on physical bodies as to convey to the material eye the sensation of sight, and from the sun to creatures on distant planets it must carry the heat force, thus giving rise to the sensation of warmth. Through this medium, yet without sensible contact with it, worlds must move, and planetary systems revolve, cutting and piercing it in every direction, without loss of momentum. And yet, as I have said, this ether must be in such close contact as to convey to them the essence that warms the universe, lights the universe, and must supply the attractive bonds that hold the stellar worlds in position. A nothing in itself, so far as man's senses indicate, the ether of space must be denser than iridium, more mobile than any known liquid, and stronger than the finest steel." "I can not conceive of such an entity," I replied.