It has been recently pointed out to me that Counterspelling may not be applicable to many Illusion spells.
Counterspelling (Spell Defense) specifies that "Spell defense is used against hostile spells cast at you or at targets that are within your line of sight (using the same rules as for targeting spells) that you decide to protect with spell defense."
However, spells like Invisibility, Physical Mask, Trid Phantasm, etc. aren't targeted at the individuals that make the defense test against the spell. Defense for these spells is based on whether or not you see through the effect which is cast on a 'third-party' target.
The same could be said for most Detection Spells, but when I looked it up, Active Detection spells actually specifies the defense has: (+ Counterspelling if available)
So, is Counterspelling useless against "passive" style illusions like Invisibility or Physical Mask? The illusions that can be resisted but aren't targeted at the opponent?