Any sufficiently advanced technology is indistinguishable from magic.
This phrase has been a keystone in one of the major arches of the architechture of speculative fiction. In a way it is what anchors the science fiction to the fantasy. (Secret: Its all fantasy. Even Tom Clancy and Robert Ludlum just make stuff up.) I have said that Swords and Sorcery is not a genre. In my mind its more of a color. I think saying you like Swords and Sorcery is a lot like saying you like red cars. Its just the color of the car and each car is vastly different than the next. But Clarke's quote has inspired things like Dying Earth literature, aspects of Sword and Planet, He-Man and lots of other fantasy paint jobs.
But I got to thinking about this statement. The first thing that occurs to me is that it is only really true in effect, not origin. That is to say a wizard with a lightening bolt spell and a guy with a lightening bolt gun have the same effect, but one guy is pointing and mumbling while the other is pointing and grunting. But even beyond that, we see that Clarke, for all his insight has not really looked at magic. Magic ignores rational cause and effect. Science, when you understand the natural principles it is based on does not. Anyone taking a look at the works of H.P. Lovecraft runs up against this whole debate. Is the spell that opens a gate to for the extradimensional horror just really alien technology or magic? Both?
Technology that appears to be magic is still just technology. Take for instance the D&D spell timestop. How are scientists spending billions of dollars to create a device to hold time static in a field indistinguishable from the witch doctor that waves a stick says some weird words and gets the same result for much cheeper?
Larry Nivin (and Mecedes Lacky) have tried to be cool and have come back with "Any sufficiently advanced magic is indistinguishable from technology." But that is just it, big magic does not necessarily require big work and power, whereas big technology does require a bigness that magic just does not require.
Lets take the atom bomb as an example. Numerous scientists and millions of dollars of research went into developing the ability to blow up an entire city in one shot. Even now the bombs are expensive and labor intensive to produce and (hope not) deliver with accuracy. Yet they still require a team of people. Whereas with magic Waldorf can take out entire planets and campaign settings with a ceremony. What separates them is cause and effect. Technology has logical cause and effect based on the natural principles of the universe it occupies. Magic ignores this. Magic in its purest form is super-natural.
Clarke's law holds so long as your story is based on two things: The first is that there is no such thing as the supernatural. All the mumbo jumbo and abracadabra is simply the ability to tap into somthing that science does not understand yet. The second is that those who experience the effect interpret the event based on their experience and knowledge. The second is immutable. It is how humans operate. But the first is an assumption. What if magic operated outside of the physical world in all aspects- what if waving a rubber ball and a feather in the air while speaking latin backwards really did summon a Ferrari? There is no physical reason a Ferrari should appear. Its not even a quirk of the universe, it is beyond what is supposed to happen.
Also, the drawbacks to magic are ussually very personal. Rangeing from colds, to being swollowed by extradimensional horrors, technology is far more expensive, but much much safer. Technology allows you to settle vasts swaths of untamed land. When magic is really magic of the supernatural kind it is too fickle. Fickle magic can't hold back the monsters enough. I think this concept can be used to explain the whole points of light campaign setting. No magic is different from technology even if it is similar somtimes in effect.
Additional: The Great Hong Ooi has a great (probably more correct) corolary:
Any sufficiently widespread magic is indistinguishable from technology