Originally Posted by Mark Hall
Which, like many things the Sage posts, contradicts that which is written in the books.
I haven't found anything "in the books" that clarifies this issue. If you know where this is addressed in any of the canon material, please share that with us.
Here's the full discussion from Dragon Magazine
#163, p. 104:
Q. How is each spell's school chosen? Also please explain how the various pairs of opposing schools of magic were chosen. What is the nature of the opposition? Why do illusionists have to contend with an extra opposition school?
A. Generally, spells are grouped into schools according to the types of effects they produce and, by extension, the type of power used to produce their effects. Abjuration blocks, dispels, or protects; alteration produces changes in its target; conjuration/summoning brings material from some other place; enchantment/charm grants its users or targets special abilities; greater divination reveals information; illusion/phantasm causes perceived changes that are not real; invocation/evocation creates matter or energy; and necromancy provides or removes the energy of life. See The Complete Wizardís Handbook for detailed explanations of each school.
No two schools of magic are mutually hostile due to their natures; opposition arises from how the spells are learned and used. Specialist wizards employ methods of study and mental discipline that enhance their abilities to use certain types of magic and erode their aptitudes for others. This is why generalists can freely use magic from opposing schools while specialists cannot. Specific pairs of opposed schools were selected according to common sense and game balance. For example, if invokers spend a lot of time learning how to bring things into being from nothing, they probably neglect to learn how to call things from one place to another. Illusionists, who spend their time trying to create believable unrealities, have a hard time casting magic that produces and channels real energies; there are three such schools: invocation/evocation, necromancy, and abjuration.
Q. I have noticed that a few spells are parts of two opposing schools of magic. For example, the limited wish spell is both conjuration/summoning and invocation/evocation. Can specialist wizards from either school use such spells? That is, can conjurers or invokers use limited wish? How is it that a spell can be from two opposing schools anyway?
A. A specialist wizard can use any spell that falls within the schools allowed to him, even if the spell in question also falls under an opposing school. Certain effects, such as limited wishes, can be produced in more than one way; in this case, what is wished for can be plucked from some other place and delivered, or it can simply be created to order.
I find this to be a well-reasoned explanation. School opposition does not imply that the schools are mutually hostile. However, it does come down to how one's DM (or you, as DM) interprets the rules in their campaign. I, for one, interpret it to mean that giving spells two schools allows for specialist wizards to have more
rather than less access to them.