From Andrew Gronosky's Ars Magica Site
Magic Resistance in Andrew's sagas does not keep out magical things like a force field. It prevents magic from affecting the magus.
Our Magic Resistance works as follows:
- Magic that doesn't affect the magus is not kept out. If a spell puts a tiny pink dot on someone's sword, the sword is not then kept from hurting the magus.
- If anything under an active magical effect crosses the boundary of the magus's Parma Magica, he knows it is magical if and only if the Penetration of the effect is less than his Magic Resistance.
- Having a Might Score does not make something inherently magical. A dragon's teeth are not blocked by Parma Magica, but its breath would be.
Some more clarifying examples:
- A sword affected by Edge of the Razor would do damage like a club because its magically-enhanced edge can't hurt the magus.
- A magically-conjured wolf could not bite the magus, because it is a magical construct and Magic Resistance prevents it from affecting the magus.
- A wolf with a Might score could bite the magus with its teeth, but any powers of those teeth would not work unless they penetrated.
- A magus who is under the effects of a Longevity Ritual or a Muto Imaginem disguise is not kept from slapping another magus.