Someone is isolated in a cell (small world) and she is familiar with scientific method. She suffers from a mental condition involving hallucinations. The problem is her hallucinations are consistent. No matter what experiment she conducts, the result is consistent with her hallucinations (no insight).
If other people are added into the cell, they all agree that some believes held by our subject are delusions. So she suspects about her mental health based on consensus of other people.
Now what if the others are not consistent in their believes? Which one our thought experiment subject should accept as reality: her consistent outlier believes or the others inconsistent ones?