There is a proverb in the Telugu language:
The hare he caught had only three legs.
He has caught a hare that has only three legs and has now convinced himself this is a rare hare, not just one that has lost a leg. He is so completely convinced about the rarity of this hare that he is unwilling to listen to anyone who says otherwise.
Do we have a similar analogy, proverb, fable, anecdote, metaphor, or story in English?
This is along the same lines as Theodore Woodward’s advice to medical students: “When you hear hoofbeats, think horses not zebras”. In other words, don’t assume you have a rare situation (hare with three legs) when there is a more common, and hence likely, explanation.
In the United States, there's a similar story/joke that exists in many versions. It emphasizes the incurable optimism of the person who finds the mundane object, and how they still manage to convince themself that it's something desirable.
A child wakes up on Christmas morning and is surprised to find a heap of horse manure under the tree instead of a collection of presents. Yet, the child is not discouraged because he has an extraordinarily optimistic outlook on life. His parents discover him enthusiastically shoveling the manure as he exclaims, “With all this manure, there must be a pony somewhere!”
In older tellings, the story of the optimistic child is sometimes contrasted with a pessimistic child who is given lots of toys and is disappointed because they'll just break.
This joke was reportedly one of Ronald Reagan's favorites; and according to William Safire, was well-known enough around the middle of the 20th century that the punchline would have been sufficient for many people to recall the entire joke.
There might be something relevant to the "hare story" in Aesop's fables but nothing comes to my mind. FWIIW, you could say that the person in question has dug their heels in on this.
Not quite what you're after and not exactly a proverb, but one of my personal favorite phrases is
I have made up my mind, please don't confuse me with facts.
You can read some more details about the phrase's origin here. It is generally used for people who refuse to accept facts that contradict their held beliefs. Granted, since it is such an obviously absurd statement, it is used more to poke fun at such people, but that makes it a good candidate for the use you mention:
He's made up his mind. This is clearly a rare hare, don't confuse him with facts.
"Pining for the Fjords" (Updated)
There is a famous British comedy sketch called the "Dead Parrot sketch" from a TV series called "Monty Python's Flying Circus" made between 1969 and 1974.
The sketch shows a man trying to return a parrot he bought from a pet shop because although sold to him as a living parrot, it is actually dead and been stuffed by a taxidermist. Throughout the sketch the shop keeper keeps trying to convince the customer that the parrot is from Norway and, in one line, is not dead and merely "pining for the fjords".
Your question reminded me of this story, with the twist that the shop keeper is only pretending he believes something to be true rather than actually believing it.
I think it safe to say English speakers are well aware of the context of the proverb and would rate the sketch as a modern day classic. Search YouTube for "dead parrot sketch" if you wish to view it. Enjoy!
A proverb used in English,
There are none so blind as those who will not see
carries the meaning of
"You will never be able to make some understand or accept something if they are too stubborn or unwilling to learn or notice." Farlex Dictionary of Idioms. (2015)
The proverb is a partial quotation of a verse in the Book of Jeremiah (Hebrew: ספר יִרְמְיָהוּ) in the Hebrew bible, and so I suspect it probably has equivalent counterparts in languages other than English.
That dog won't hunt
That idea or proposal is not viable or has no chance of succeeding; that won't work.
Farlex Dictionary of Idioms
(US) Phrase. That idea will not work; that is an inadequate explanation or proposition.
Wiktionary
With reference to a dog used for hunting game, the colloquial American-English phrase that dog won’t hunt, and its variants, are used to express the opinion that a particular plan or approach will not succeed.
—Synonym: that cock won’t fight.
wordhistories.net
Pieter Bruegel the Elder's 1559 painting of Netherlandish Proverbs aka The Topsy Turvy World is a treasure trove of sayings about human foibles.
There are about 126 identifiable proverbs. While I couldn't find a one-to-one correspondence with the OPs specific theme, there are some close equivalents, e.g.:
004: "To bang one's head against a brick wall" (i.e., to waste one's time on an impossible task)
042: "To shoot a second bolt to find the first" (i.e., to repeat a foolish action)
In American English, we might say "that's the hill he wants to die on" painting the picture of a battle between himself and the forces of reason who oppose him. He's defending his hill of opinion from those who seek to conquer him.
He is on a quixotic mission. Notice that in modern English the adjective quixotic seems to have two slightly different meanings: (1) idealistic or unrealistic; and (2) pursuing a futile or illusory goal. Here I am referring to the second meaning.
The issue of the double connotation was discussed here.
He is being "stubborn as a mule" would match his refusal to change his mind despite the opposition from others - it does not cover his initial error. See Collins Dictionary
One might say he is in "Blissful ignorance" - but that would be better if he was genuinely unaware of his error rather than being obstinate. See Collins Dictionary
If he was clinging onto his belief, unable to understand what others were explaining, you might say "The lights are on, but nobody is home" See Collins Dictionary
One could say "He's covered himself in lemon juice and intends on robbing a bank."
I have derived this sentence in reference to "The Dunning-Kruger Effect" which is an interesting case to read up on. To paraphrase the conclusion of the study it was found that when someone is left to their own devices without an alternate perspective it can have unintended results, sometimes of a dangerous nature.
The conclusion was based on a perfectly sane gentleman who has watched a documentary on invisible ink. His hypothesis was that lemon juice could make oneself invisible. To test this he applied lemon juice to his face and body and took a Polaroid selfie. Upon looking at the developed photo he was missing from it, and it provides him with all the empirical evidence he needed that his idea was correct. Armed with this knowledge he had obtained he proceeded to rob a bank wearing nothing but lemon juice.
The doctors assigned to his case after he passed a psych eval did further testing and found he probably just held the camera wrong, but didn't have anyone there to give him further perspective which ultimately left him with his false beliefs.
The doctors published their findings and coined it appropriately with their names as they were the first to document this extreme case and example of The Dunning-Kruger Effect.
A similar concept as the three-legged hair is fool's gold (pyrite), a mineral that looks like gold, but is worthless. Metaphorically, it's used to describe something someone thinks is worth a lot, but isn't, i.e. "This bitcoin is nothing but fool's gold."
When an artifice failed him he claimed credit for artistry !
"Sunk Cost Fallacy"
When someone will cling to a belief or action, despite it being shown to be irrational.
"Confirmation bias"
People display this bias when they select information that supports their views, ignoring contrary information
"Double down"
To continue to do something in an even more determined way than before (even when wrong).
