There's some good sense in this thread but the initial premise (Clarke's Law) and some statements here are utter hogwash.
Red Wyrmling said:
But then again being reliable untill proving not to be is something that goes for technology too.
So therefor I state: MAGIC IS TECHNOLOGY.
Though this has nothing to do with Clarke's law it is actually quite insightful. It is no coincidence that the Renaisance alchemists and magicians were often the same people as the early scientists. Nor is it any coincidence that Dr. Faust, through his magic gained a more than a few technological advances. (IIRC, in Marlowe's play, he was given gunpowder). Both magic and technology are the product of the desire for power over nature. Magic was the largely unsuccessful project for power over spiritual nature while technology was the wildly successful project to gain power over physical nature. Modern medicine differs from a witch doctor's medicine in technique but not in its goal--nor in the effects that give it credibility. (Like the surgeon whose patients die in surgery, a witch doctor whose activities never effected any apparent cures would have little credibility).
Thus, while parts of the Forgotten Realms and even higher-magic settings may seem somewhat silly or even Harry Potteresque in their use of magic to substitute for recognizable technology, they are actually quite plausible. Their lack of plausibility comes from the fact that they generally approach this in a haphazard manner and that they generally assume a 20th to 21st century western role for technology--producing comfort and affluence for the majority of the population--rather than a more ancient and typically non-western role for technology--securing and expanding the power and comfort of a ruling class. (Keep in mind, when thinking about this that many of the technologies that currently secure the power and comfort of the ruling class in places like North Korea are essentially exports from places where different social and priority structures allow and encourage creativity and development. Cell phones, for instance are popular in Saudi Arabia but Saudi Arabia did not invent them, does not manufacture them, and left to its own devices or in a world full of Saudi Arabias, would not have them). That's why I find the highly magical wizard's tower built by elementals and demons where dominated slave girls provide entertainment, golems stand guard, animated fans provide air conditioning without the need for slaves to power them, and summoned imps perform menial tasks more credible than the city with magical mass transit. In the first scenario, magic is employed to perform the functions of technology to provide for the comfort and power of its possessor. In the second, it's employed to provide for the comfort and affluence of the population in general.
If a medieval dude ends up in New York City he'll see magic all about, not technology. Vice versa, if an engineer from New York would end up in a fantasy world where he witnessed magic he would try to find a reasonable, scientific explanation (or wonder what he's on this time!).
Therefore I also state: TECHNOLOGY IS MAGIC.
No, the medieval man wouldn't see "magic" all about him. To maintain such is mere chronological snobbery that ignores the rational nature of the medievals. When faced with reports of a rocks that burn (coal IIRC) and cloth (asbestos) that didn't from far lands the medievals did not look to magic for an explanation of these phenomena but considered them to be strange but natural things. The medieval era saw the early development of a lot of technologies that went on to mature in the early modern and industrial eras. Water clocks, for instance, were very complex and their workings were opaque to the common man, but I'd be surprised if anyone thought they were magic.
In fact, if one is willing to gloss over the differences between magic and miracle (which is probably appropriate in this context), the events of the Reformation clearly demonstrate that late medieval/early modern people were able to distinguish between magic and technology. (I don't have my references on me at the moment so I hope I get the names right but this was not a unique event in the Reformation so even if I get some of the names or locations wrong, it's still relevant). One such event was English reformation preacher, actor and playwright John Bale's demonstration of the fraud of an English Rood screen. Until that point, it had been thought that the rood screen bleeding blood was a miracle. When John Bale exposed the hidden mechanisms that caused the rood screen to appear to bleed, it became apparent to all the observers that it was simply technology.
This indicates an awareness of the difference between technology and magic. That difference, I submit lies primarily in the context and the nature of the forces purportedly harnessed. The rood screen initially appeared to be magical/miraculous while water-clocks did not because it appeared in a context and had a nature that led people to believe that its causes werre spiritual rather than mechanical or natural. (This probably gained credibility because of other, similar miracles that had appeared to be genuine upon closer inspection thus creating a genre of bleeding statue miracles just as there is currently a genre of "image of the virgin" miracles). When the rood screens' mechanisms were revealed, it became apparent that, unlike a genuine instance of magic/miracle, the causes of the strange phenomena were mechanical.
Unless the context and function of a technology were apparently miraculous or magical, it is unlikely that a medieval would think it to be magical/miraculous. And if the context and/or function of the technology were such as to lead one to believe in a miracle, it would fall under the category of deliberate deception rather than natural appearance. But then, it should come as no surprise that people can and often do use technology to deceive people and if all that Clarke's law is taken to mean is that people with sufficiently advanced technology can trick those unfamiliar with such technology, it is pretty much content free.
