ISTROPY is a physical/mental characteristic, thought to be set at birth, that partially determines the ability of people to cast magic spells. The physical/neurological causes of high (and low) istropy are not understood, thought magical spells have been devised (Detect Istropy) to measure the magical abilities of people. Use of this spell has revealed abnormally high istropy in certain animals, but, in general, beasts are completely free of this characteristic. Detect Istropy's effects are distorted by the caster's istropy (it is a new spell) and so this effect must be subtracted out. The whole process requires repeated testing of casters and complicated mathematics to figure out from the readings what everybody's istropy must be. The wizards, after this process, can use the spell to evaluate the istropy of others.
I.T.C. see Ilican Trading Company