I agree that this person is going to ruin your game. Youwill have to start more than doubling your work load each game. Spot checks for the party, and spot checks for the insane character,then what the part sees vs. what the insane character sees etc.... Also considerthis...if your adventuring frined whom you've known for a little while now starts acting very strangley or even a bit mad...what do you as a friend do? You take him to priests but the source of his illness isn't some kind of evil magic, it seems natural and it's geting worse and worse. Well, now you can't rely on him in combat anymore as he keeps attacking things that aren't there or spotting things that aren't there and ignoring things that are....eventually enough is enough and it is time to put your frined some place safe where he can't hurt himself....and find a new ranger to take his spot.