In reply to Link (msg # 135):
If you were to cast Mental Shield after being affected by a spell, it would help defend against any further attempts to mess with your mind, but in order to dispel an active effect you would need to use the Prime 1 Dispel spell in conjunction with the arcanum for the spell you'd like to dispel (in your case casting Dispel while having at least 1 dot of Mind), or you can also use Prime 4 Supernal Dispellation.
MtA, p.208:
The mage subtracts one die per dot he possesses in the Mind Arcanum from spells or supernatural powers that attempt to mentally control, detect or influence him. By spending one Mana, the Duration can be made to last for one day. Most mages cast such a shielding spell at the beginning of the day, as part of their morning rituals. Successes are used to combat attempts to dispel the shield.
In addition, the Mental Shield helps its caster see through mental illusions such as Misperception or Imposter. Upon first encountering such a spell, the user can see through the illusion with reflexive and contested Gnosis + Mind roll. This spell can be cast at any point in a turn, even if the caster’s Initiative would have him act later than an attacker. In this way, it is similar to a dodge action.
That last bit means that you could've cast Mental Shield as a Reflexive Action in response to me asking for a Gnosis + Composure roll, especially since your Unseen Senses and having Third Eye active would've alerted you to the fact that some sort of spell effect was incoming.
