Vanilla is a product of Lussumo:Documentation and Support.
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Type 1 impossibilities Impossible today, but do not violate the known laws of physics. Might be possible this century or the next: force fields, invisibility, phasers and death stars, teleportation, telepathy, psychokinesis, robots, UFOs and aliens, starships, antimatter and anti-universesType 2 impossibilities Technologies that sit at the edge of our understanding of the physical world. May be realised millenia or millions of years in the future: faster-than-light travel, time travel, parallel universesType 3 impossibilties Technologies that violate the known laws of physics. If they turn out to be possible, they would represent a fundamental shift in our understanding of physics: perpetual motion machines, precognition
Not arbitrary at all -- precognition violates vastly more physical laws than the other two.