Personally, my magic follows definite laws, many of which are the same as any other kind of energy--cannot be created or destroyed, etc--and constant. I have two specific kinds of magic: one is the kind my elves, mages and "psychics" use, which is quite easily defined, explained, enacted on (if one has the talent) and is somewhat akin to solar or hydro-energy.
The second is probably not quite magic. It's the kind that's acted on through prayer, meditation, etc. It is difficult to predict, often overlooked, impossible to study and quantify and thus, rather subjective. ^^ Same as prayer in real life: did you pass your exam because you prayed or because you studied/got lucky? Within my story, a Wiccan's magick falls under this category.
The only thing that's definite about the second kind of magic is that it fogs and obscures the power flows of "normal" magic in it's direct area.