From the introduction to Identification Magics: A Beginner's Guide to Identification And Detection Cantrips And Spells, currently in use in the freshman curriculum, Halruaan Academy for the Mystic Arts...
...which brings us to Detect Magic, a simple first-order spell every competent mage learns quite soon in his career. This minor but extremely useful spell allows the user to not only determine whether or not a given object is magical, but with practice, an experienced wizard can determine the intensity of the object's dweomer as well as its type of enchantment, be it necromantic, evocation, illusion, and so forth. What many students of magic fail to consider is this: how does this simple spell work?
The fact of the matter is "We don't know." The spell is old enough that its originator's name is lost in the mists of time. A clue, however, is provided in what we DO know about the spell:
*It functions only for the caster
*It is not dependent on sight
*Multiple objects may be "scanned" at once
It is theorized by Hattersley the Studious that the spell acts not upon the items in the spell's range, but upon the mage casting the spell -- allowing for a new sense, similar to sight, hearing, or smell -- a "magic sense" -- to begin to function, and to continue for a short time. Hattersley is also noted for his "Third Eye" theory -- the idea that several detection and analysis spells are tied to one's vestigial "third eye," and allow it to "open" and percieve the universe "differently" for a short time (i.e., the spell's duration).
While this theory is interesting, it begs the question of what would happen in a situation where to percieve LESS would be desirable -- say, in the presence of a medusa or catoblepas. What horrors might a "third eye" reveal if it were to see too much?