On what basis can we claim somebody is delusional? Assuming objective, True Reality(tm) exists, but is not directly knowable and is only knowable through mediation of our senses, how do we have any solid footing for deciding one person's senses are defective compared to another's? Two thought experiments to illustrate this idea: Assume I am alone in a room, and I see a purple monkey swinging from the lamp. I perceive this odd sight and may, or may not, decide that I'm hallucinating based on my previous experiences. If somebody else comes into the room, and I ask them what they see, if they agree with me, then odds are better that we are both seeing accurately, but if he disagrees with me, then he may be blind to the monkey, or I may be imagining it. Adding more people will get us a consensus view, but doesn't really prove anything in more than a statistical way. Who is delusional, and who is seeing truly? Or, assume I am the one-eyed man in the land of the blind. Like the monkey-seeing fellow in the previous example, I perceive odd, starnge things those around me don't. From the previous statistical argument, I'm just as likely delusional as the monkey-fellow, but in this case, I'm actually seeing something my compatriots are literally blind to. This would imply that "group opinion" is insufficient to discern hallucination from enhanced perception. So, how should such a determination be made?

There seems to be a pattern of argument here that needs to be questioned. It is: (i) Method M for reaching judgements isn't completely reliable; therefore, (ii) method M can't be trusted. The conclusion simply doesn't follow. Method M might be very reliable, in which case it can be trusted to a high degree.

I doubt "group opinion" is the only method available to us for distinguishing hallucination and illusion from perception. (For example, in the land of the blind, there might be ways of correlating your apparent perceptions with facts on which you can all agree, e.g., that there is a boulder in such-and-such a location that wasn't there yesterday.) But it might be quite a good way of drawing the distinction even if it isn't a perfect way.

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