In a true
magical system--a real, practicing
magic system, not an RPG system--"white" magic does not
exist. Say 'white magic' and everyone thinks of a perfect,
pure, virginal, idealistic
magic with
flowers and
unicorns and such. No such luck. (The practitioners, btw, who tend to bill themselves most loudly as 'white magicians' usually have the
weakest understanding of what they're actually
doing or
believing, either through honest
ignorance, lack of
research, or ...
questionable influence...)
Beneficial, positive magic does exist. Pure, perfect "white" solidly light magic does not. The entire concept of a magical system is one of balance. Yin and yang, masculine and feminine, light and dark. (It's this same sort of reasons alot of practitioners question dianic wicca, since there's a use of ONLY female energy and a shunning of everything male.) Balance is the key--even in the wheel of the year, half the year is classified as dark and half light. Balance just IS.
One may perform magic with good intent and no ill wish of harm--indeed this is EXPECTED in most systems, such as Wicca, where the main principle is "An harm ye none do what ye will". But any spell, no matter how well-intended, CHANGES things in some form. It has some influences, good or bad. And the larger the spell the larger the concequences on both sides.
You may cast a spell, for example, weather witching to keep the night clear for your coven's ritual. But could be someone else near you needed the rain for another purpose, maybe their crops, or maybe the need aged water for their fishtanks. You did no intentional harm towards this person, but there was still some negative reprocussion, and the presence of any 'dark' implies the 'white magic' is impossible. You may do a spell to turn away the unwanted attentions of someone who's stalking you. This is meant only as selfpreservation, but this follower can then make someone else's life as miserable, having lost his focus on you he finds another. In this world, a perfect, pure system cannot be. Everything touches everything else too fully.
If you practice as a pagan, wiccan, witch, or any other form of magic, i can only suggest (and hope) you choose to practice beneficial, positive magic. But leave your white magic in your RPG's and fantasy novels wehre it belongs.