Who Stole What from Whom? (Who Stole What from Whom? Part X)

(N.b. my reasoning differs from that in the solution presented by the author substantially, yet we reach the same conclusion. I am not sure the solution is airtight but it seems pretty solid so I am posting it here anyway.)

“And now, we come to a particularly good puzzle,” I said to the group proudly.
“Three girls—Abigail, Bernice, and Carol—each had a pet; one was a dog, one a cat, and the other a horse, but we are not told which girl owned which pet. One day, our three villains—Mike, Spike, and Slug—each stole a pet from one of the girls, but it was not known who stole what from whom. The case proved extremely baffling, but, fortunately, Inspector Craig of Scotland Yard was visiting the country at the time...”
“Who is Inspector Craig?” asked Barry.
“He is a character from one of my books,” I replied.
“What is the name of this book?” asked Barry.
“You just guessed it!” I said.
“Whatever do you mean?” asked Barry in astonishment.
“I mean just what I said; its name is What Is the Name of This Book?”
“Stop kidding us!” said Barry.
“He’s not kidding!” said Alice. “I’ve read the book, and its title really is What Is the Name of This Book?, and it really does contain a whole chapter of cases from the files of Inspector Craig.”
“Anyway,” I intervened, “Inspector Craig was able to find out the following facts, which were enough to solve the case.

1. The one who stole the horse is a bachelor and is the most dangerous thief of the three.
2. Abigail is younger than the girl who owns the dog.
3. Mike’s brother-in-law, Slug, who stole from the eldest of the three girls, is less dangerous than the one who stole the dog.
4. The man who stole from Abigail is an only child.
5. Mike did not steal from Bernice.

Who stole what from whom?”

(Source: King Arthur in Search of His Dog and Other Curious Puzzles by Raymond Smullyan)

Slug is revealed to be the brother-in-law of Mike, which means that Mike is married and is therefore not a bachelor and is therefore neither the horse-thief nor the most dangerous of the three. Additionally, Slug has been described as "less dangerous" than one of the other thieves, which leaves only Spike to be the horse-thief.

Since the horse is now (partially) accounted for, it should be pointed out that Slug is compared as "less dangerous" to the one who stole the dog. This leaves only the dog to be stolen by him. And it also means that Mike stole the dog. So far, so good; who stole which pet is already fully accounted for.

Slug (the cat-thief) stole from the eldest of the three girls. Since Abigail is described as younger than the girl who owns the dog these facts mean jointly that she must own the horse.

With the horse now fully accounted for, the fact that Mike (the dog-thief) did not steal from Bernice means that Bernice owns the cat. With only one pet left, Carol must own the dog.

Final answer: Mike stole the dog from Carol; Spike, the horse from Abigail; and Slug, the cat from Bernice.

Who Stole What? (Who Stole from Whom? Part IX)

One day, Mike, Spike, and Slug went to the neighboring town of Middleberg and committed three robberies. One of them stole a rifle, one stole some money, and one stole a book. The three were caught, but it was not known which man stole what. At the trial, they made the following statements:
Mike: Slug stole the book.
Spike: Not so; Slug stole the money.
Slug: Those are both lies. I didn’t steal either!
As it happened, the one who stole the rifle was lying, and the one who stole the book was telling the truth.
Who stole what?

(Source: King Arthur in Search of His Dog and Other Curious Puzzles by Raymond Smullyan)

Mike cannot have stolen the book, because he would have said that he stole the book. Slug cannot have stolen the book for the same reason. Therefore, Spike stole the book, which means that Slug stole the money and Mike stole the rifle.

Who Owns the Goat? (Who Stole from Whom? Part VIII)

The goat belonged to either Farmer White, Farmer Brown, or Farmer Black. Farmer White claimed that the goat was his. Farmer Brown claimed that the goat did belong to Farmer White. Farmer Black either claimed that the goat belonged to him, or he claimed that it belonged to Farmer Brown, but, unfortunately, the court records are confused on this point. At any rate, at least two of the claims were correct.
Who owns the goat?

(Source: King Arthur in Search of His Dog and Other Curious Puzzles by Raymond Smullyan)

If Farmer Brown owned the goat, then Farmer Brown would be lying, as would Farmer White, leaving only one more claim to be possibly true, so Farmer Brown does not own the goat. If Farmer Black owned the goat, then Farmer Brown and Farmer White are still both lying. Therefore Farmer White owns the goat.

Who Stole the Goat? (Who Stole from Whom? Part VII)

One day a goat was stolen. Naturally, Mike, Spike, and Slug were the suspects, and, in fact, one and only one of them was guilty. Each of the three accused one of the others, and Mike was the only one who lied. Was Mike necessarily guilty?

(Source: King Arthur in Search of His Dog and Other Curious Puzzles by Raymond Smullyan)

Consider the two alternatives: first, if Spike stole the goat, then Spike cannot accuse himself, only one of the others, falsely. The same contradiction applies to Slug. Therefore Mike was necessarily guilty.

Which Farmer Owned the Horse? (Who Stole What from Whom? Part VI)

The horse was recovered and was to be given back to the rightful owner, who was either Farmer White, Farmer Brown, or Farmer Black. The three farmers each made two statements:

Farmer White:

1. The horse does not belong to Farmer Brown.
2. It belongs to me.

Farmer Brown:

1. The horse does not belong to Farmer Black.
2. It belongs to Farmer White.

Farmer Black:

1. The horse does not belong to Farmer White.
2. It belongs to me.

As it happened, one of the three made two true statements; one made just one true statement; and one made statements that were both false.
Who owns the horse?

(Source: King Arthur in Search of His Dog and Other Curious Puzzles by Raymond Smullyan)

Assume that Farmer White is making two true statements. Accordingly, Farmer Brown is also making two true statements, which is a contradiction. Conversely, the assumption that Farmer Brown is making two true statements is rendered a contradiction by the statements of Farmer White, who would also be required to make two true statements under this assumption. Therefore, only Farmer Black can be making two true statements.

It has now already been established that Farmer Black is the owner of the horse but to complete the puzzle, it should be pointed out that Farmer Brown is making two false statements and Farmer White is making one true and one false statement.

Who Stole the Horse? (Who Stole What from Whom? Part V)

One day, a horse was stolen. Again, Mike, Spike, and Slug were rounded up for questioning. This time, each one made two statements. None of them made more than one false statement.

Mike:

1. I did not steal the horse.
2. The one who stole the horse is Italian.

Slug:

1. Mike never stole the horse.
2. The one who stole the horse is German.

Spike:

1. I never stole the horse.
2. It was Slug who stole the horse.

Assuming that one of those three men really stole the horse, which one was it?

(Source: King Arthur in Search of His Dog and Other Curious Puzzles by Raymond Smullyan)

Spike cannot have stolen the horse because he would be issuing two false statements. If Mike stole the horse, then his second statement must be true, namely that the one who stole the horse is Italian. But then Slug would be issuing two false statements as was the case with Spike earlier. Accordingly, Slug stole the horse.

Who Owns the Cat? (Who Stole What from Whom? Part IV)

The cat belonged to one of three girls—Annabelle, Betsy, or Cynthia. Annabelle claimed that Betsy doesn't own the cat, and Betsy claimed that Cynthia owns the cat. Now, it so happens that the the owner of the cat always tells the truth and is the only one of the three girls who ever tells the truth.
Who owns the cat?

(Source: King Arthur in Search of His Dog and Other Curious Logic Puzzles by Raymond Smullyan)

Suppose that Annabelle is not the owner. This would imply that her claim that Betsy doesn't own the cat is a falsehood. This would in turn imply that Betsy is the owner of the cat and therefore speaks truth. But if Betsy were the owner of the cat and accordingly a truth-teller, she would not claim that Cynthia owns the cat, which is a contradiction. Therefore Annabelle must be the owner of the cat.

Who Stole the Cat? (Who Stole What from Whom? Part III)

One day a cat was stolen. Mike, Spike, and Slug were again rounded up for questioning. Mike claimed that Spike had stolen it, and Spike claimed that Slug had stolen it. Now, it was not certain that any of the three suspects had stolen it, but later investigation showed that no guilty person told the truth and no innocent person lied. Also, the cat was not stolen by more than one person.
Can it be determined who stole the cat?

(Source: King Arthur in Search of His Dog and Other Curious Puzzles by Raymond Smullyan)

If Mike stole the cat then Spike's accusation is false, which is not allowed, letting him off the hook. The same reasoning applies to the possibility of Slug being the thief: Mike is falsely accusing Spike. It can't be the case that no one stole the cat because in this case, not one but two false accusations are being made. Therefore Spike stole the cat.

Who Owned the Dog? (Who Stole What from Whom? Part II)

The dog was recovered. It belonged to one of three boys—Arthur, Bernard, or Charles. They made the following statements:
Arthur: Bernard doesn't own it.
Bernard: That is true.
Charles: Arthur doesn't own it.
As it happened, the real owner was telling the truth, and at least one of the others was lying.
Which boy owns the dog?

(Source: King Arthur in Search of His Dog and Other Curious Puzzles by Raymond Smullyan)

Charles is excluded from being the owner because that makes the other two statements true. Bernard is also excluded because he would be agreeing with a lie (Arthur's statement). Arthur is the only candidate left.