Defensive Casting while Blind

[Roaring Viking]

Defensive casting forestalls attacks of opportunity. So, can you defensively cast if you cannot perceive the attack of opportunity (or the attacker). For example, if your are blind, the attacker is an unsensed invisible attacker, etc? How would you prevent the attack of opportunity from happening if you cannot see it?

[Java Man]

You are keeping your gaurd up to avoid leaving an opening to get sucker punched, you're not preventing the attack. This explanation helps me rationalize how the rules work at times like this.