PDFs of answers to most of the exercises in IFL are linked below. (Do let me know of errors, by adding a comment below.)

### 14 Responses to Answers to exercises

1. Igor Stojanovic says:

Concerning exercise C from chapter 14, It seems to me that the argument 10. from previous exercise can also be considered as problematic when translated using “if … then …” for the reasons explored in chapter 15, as it has “not-P or Q” among its premisses, and the conclusion is in the form “if P then Q”. Consider:

Either Wordsworth didn’t write “The Lord of the Rings”, or he wasn’t a novelist.
It isn’t the case that Wordsworth wasn’t a novelist and that he wrote at least one novel.
So, if Wordsworth wrote “Lord of the Rings”, he wrote no novels.

Here , it seems that if the first premiss is endorsed on the basis that Not-P is asserted, the material conditional in the conclusion won’t be robust.

2. Charlie Wood says:

Just to correct a minor error in your 4th printing (2011) – in your given online answer to Question 7, section D, Exercises 7, I think the ‘Qs’ should be replaced by ‘Ps’. Correct me if this supposed ‘error’ is in fact no such thing!

Hello,
Just wanted to let you know of a mistake in the answers for exercise 29.B part 7 in line (4) you write “Kyn” and then later you rewrite it as “Kxn” I don’t know which of the variables you meant it to be but I’ve tried the tree with Kyn and it leaves an open branch so I’m guessing it must be Kxn.
Great book!

4. Isabel Allard says:

Exercises 16, Question 6, has Q \/R in the textbook, but the answers has Q^R; the difference between ‘and’ and ‘or’ being somewhat important at times, I just thought I’d mention it…

Stay logical,
Isabel

5. FlyingHamster says:

On Exercise 13 C6, if A is a contradiction, there would be no valuation which makes A true (has it false on the answers), since A is false, there would be no valuation B which would allow A|=B.

• Antonio Ramos says:

I had the same problem with this answer, and it seems wrong. Indeed, it seems to contradict the explanation given in the book for (v) “If C is a contradiction, and A1, A2, …, An, B tautologically entail C, then A1, A2, …, An, tautologically entail ¬B.” (p. 121). There it is clearly stated that “if C is a contradiction, C is never true on any valuation.” But then in the answer to exercise 13 C6, Smith says “if A is a contradiction, then there is no valuation which makes A false.” Something is not right here.

• Peter Smith says:

Yes, “false” was a typo for “true”. Sigh! Now corrected (and apologies for the oversight in not correcting before in response to FlyingHamster).

6. Philip Papagiannopoulos says:

Also, in the same set of exercises (16), exercise 5, the last premiss is (~R\/S) at the textbook but the solution given is for (~R^S).

By the way, Prof. Smith, I’ like to say a big ‘thank you’ for your excellent book.
Greetings from a student in Greece.

Respectfully yours,
Philip Papagiannopoulos

7. Ari Krupnick says:

In the answers for exercises 27, it says that wff (7) is false on the second q-valuation that is provided. But (7) is true on that valuation. The number 4 is an even number in the domain which is less than every odd number in the domain.

• Peter Smith says:

8. Derek Scott says:

In the answer to exercise 8A – 6, I think the answer isn’t quite fully expanded, with a not(S) left on the left hand side.

• Peter Smith says:

Oops, yes! Thanks for spotting that. Corrected on the server now (the new version may take a little while to show up).

9. Guilherme Simões says:

There is a really minor mistake in the solution of the exercise 2 in chapter 3. Instead of “Some humans AND men” should be “Some humans ARE men”.

10. Guilherme Simões says:

Also in chapter 3: Exercise 5 of the solutions is different from the book.