It is easy to confuse Illusion and Alteration. Both schools of magic attempt to create what is not there. The difference is in the rules of nature. Illusion is not bound by them, while Alteration is. This may seem to indicate that Alteration is the weaker of the two, but this is not true. Alteration creates a reality that is recognized by everyone. Illusion's reality is only in the mind of the caster and the target.