Thank you very much,
What about the side question:
What interest would someone have in claiming a thief defense when he could instead claim a robber defense and get all the benefits of a thief defense without the double payment?
Best regards,

Let me preface that Maimonides in Laws of Theft ch. 4 defines an armed robber as a robber in secret, meaning a person who takes by force but not in public. So someone who claims an armed robber defense is really making a kind of thief claim, even though we are speaking about compulsion.
And as for your question: a robber defense is a risky claim, because robbery is done in public and it is easy to refute his claim. Many people should have seen such a robbery. Therefore custodians usually will not claim a robber defense, because they are afraid of being contradicted, and prefer to lie with an armed robber defense. But precisely for that reason, the claim of an armed robber is treated as a thief claim that incurs double compensation, even though it involves compulsion.
By the way, from here it is also clear why one who makes a thief claim is like a thief and is liable for double compensation. He is trying to evade the depositor by means of a claim about something done in secret, without taking the risk that his claim will be disproved. That is exactly like a thief, who comes secretly and takes no risk.
Since there is little risk in this lie, people could easily make a thief claim without much risk of being found out, and that is why the Torah imposes double compensation on him as a penalty, in order to deter him from doing this. If he is caught anyway, he will pay double. In other words: this penalty keeps the expected loss high enough—the chance the lie will be discovered multiplied by the loss when it is discovered. Since in a thief claim (or an armed robber claim) that chance is low, the Torah makes the loss large and preserves the expected cost.

Hello, this would seemingly also be true if he claimed a lost-item defense. If so, what is the distinction between a thief claim and a lost-item claim?

Maybe the same law would apply even to one who makes a lost-item claim.
But if so, then someone negligent in secret would also have the same law, and that does not seem reasonable.

One who makes a lost-item claim is exempt from double compensation. Is there any explanation for the distinction (aside from it simply being a scriptural decree)?

I don’t know. Jewish law defines one who makes a thief claim as a thief himself. It does not look like liability for the claim itself, but rather that there is a definition that he himself is considered a thief and is liable as a thief. And one who makes a lost-item claim is not considered a thief. But of course that is not an explanation. The question is why one who makes a thief claim is liable for double compensation, not why one who makes a lost-item claim is exempt from it. Once we understand that, maybe we’ll understand the other one too.

“The question is why one who makes a thief claim is liable for double compensation.” What is the question? He is liable for double compensation exactly like any other thief who quietly made the object disappear from the owner’s possession. Why say that “it does not look like liability for the claim itself, but rather that there is a definition”?

And why isn’t one who makes a lost-item claim liable for double compensation? That can be explained by the ease of the offense (which affects how common it is)—a story about theft is easy to tell, and it is hard to verify the reliability of the custodian. “The vase was here in the room, and in the morning I came and saw that it was gone; that’s all I know.” But a lost-item claim is odd—where was the object last, after all it has no legs (and if it does have legs, like an animal, you can check whether the neighbors saw it), and would you object if we turn your whole house upside down looking in every nook and cranny, and so on. Therefore a lost-item claim is rarer, because it is hard to withstand all the questioning he will go through and produce a plausible version, and the listeners are suspicious and he is embarrassed. Therefore, just as a robber is exempt from double compensation, so too one who makes a lost-item claim is exempt. Both are not so common, and therefore there was no need to penalize them as severely (the punishment is meant to reduce the frequency of the offense).

I disagree. According to your approach, there is no need for the novelty that one who makes a thief claim is liable for double compensation. It is just like any other thief. According to that, there would be double compensation for anyone who takes another person’s money or property (at least in secret), and that is not so.
As for the explanation, I’m far from convinced.

Hello Rabbi,
There is an internal contradiction in the answer given above—if he is liable for double compensation like any thief, then seemingly there is no difference between the claims. And if this is a law meant to reduce the frequency of the offense, then there is indeed a novelty in his becoming liable for double compensation.
But if we adopt the second explanation, there is some reasoning here. Similar to what Rabbi Michael wrote earlier about one who claims a robber defense, so too one who claims a lost-item defense would involve low frequency.
But in the case of one who makes a thief claim, he is not really a thief; rather, the Torah imposed on him the “law of a thief” as a kind of deterrence. Somewhat like the law of “as he plotted,” where even though he did not commit a certain offense, he receives the law of the offense that he effectively tried to invent.

And why

And why should a borrower who claimed that it died through normal work use, and it turns out he lied, not pay double compensation like an ordinary thief?