Partway through, I had the urge to look up a past comment saying something like "I've seen philosophers argue, in apparently total sincerity, whether a man in a desert seeing a mirage of a lake that coincidentally has a lake just beyond it "really" knows the lake is there".

Unfortunately I can't find it now; it probably either didn't use the exact word "mirage", used another metaphor entirely, or was actually on OB. Searching "mirage" brought up a similar metaphor in Righting a Wrong Question, but that's making a different point.