"A hallucination is a fact, not an error; what is erroneous is a judgment based upon it"

This quote by Bertrand Russell is referring to the idea that hallucinations are real experiences, not mistakes. He is stating that it is not the hallucination itself that is incorrect, however rather the judgement that is made based upon the hallucination. This is since hallucinations are subjective experiences that can be translated in a different way by different individuals. Russell is suggesting that it is important to be aware of this and to not make presumptions about the credibility of a hallucination based on one's own judgement. He is stressing the value of comprehending that hallucinations are genuine experiences and ought to not be dismissed as errors.