This is an example of magic from fantasy literature - in this case Tolkien. Many other authors have created similar specialized traditions for their own worlds.
Istari are not learned old men - they are immortal spirits who have come to this world for a cause, adopting the guise of old men for the purpose. But the other sorcerers of the world, Elrond, Galadirel, and perhaps Denethor, the steward of Gondor, obey similar rules. So does Sauron and the Witch-King, after their own manner.
Examples and References
The Hobbit, Lord of the Rings.
Limitations
Faith
The istari are servants of the maia, sent to Middle-earth to teach and to provide leadership, advice, examples and inspiration. Each of them does this in his own way, but they all serve this greater purpose at all times. They do have a great nemesis - the fallen maia Morgroth and his contemporary servant Sauron. Straying even a little from the path of righteousness is likely to lead to corruption, and any blatant use of magic risks drawing the attention of the servants of evil.
Once you have fallen, there is very little chance of redemption, and the only options is a sad death and eternal servitude to the powers of darkness. You may not imediately fall down and worship Sauron, but your days grow dark, your spirit loses its vigor, and you can no longer experience joy or appreciate simple pleasures - leaving you with nthing but the pursuit of survival, power and wealth to fill your time.
Focus
The istari themselves use staves as foci, and the rings of power can be foci as well for the lucky few who have them. Denethor used his Palantir as a focus for a highly-developed divination skill, but may have had some influence magic as well.
Common Schticks
- Artificing
- Divination
- Healing
Rare or prohibited Schticks
These prohibition reflects more on how magic works in Tolkien's work, than on any limitations on the istari.
- Flux
- Gate
- Law
- Summoning
- Time