Can one have delusional knowledge?

Depends on what you mean.

If "delusional knowledge" is supposed to mean that what the person "knows" isn't true, then the usual answer (with which I would agree) is no. We can't know what isn't so.

If "delusional knowledge" means beliefs produced by the person's delusion, but that happen by luck to be true, the answer is no according to most philosophers. The problem is that even though the belief is true, it isn't connected to the facts in the right way. To put it a bit too simply, the fact that what the person believes is true doesn't have anything to do with the fact that they believe it; they would believe it even if it were false.

If the question is whether a person who suffers from delusions can know some things, the answer is yes. A deluded person might know her own name; he might know where he lives; she might know that hydrogen is the first element in the periodic table. But due to his delusions, he might believe that astral beings are whispering the secrets of the universe in his ear. That's not knowledge.

There may be other possibilities, but you get the idea. First, say more clearly what you mean by "delusional knowledge." Then ask if it covers cases where the person holds a true belief that has a reasonable justification or a reliable connection to the facts. If so, it will probably count as knowledge. If not, it probably won't.

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