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Philosophy

1002

James Hildebrand

Spring

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The University of Western Ontario
London Canada
Department of Philosophy
Philosophy 212 (001) UW – Introduction to Logic L. Falkenstein
Mid-Year Test Wednesday, December 14, 2011
3 hours 7:00pm, EC 2155
Closed Book Page one of one page
Answer All Questions
1. Explicate in terms of open and/or closed truth-trees: 6%
a. Truth-functional consistency A set, Γ, of sentences is t-f consistent if and only if the tree for Γ
has a completed open branch. (Marks off if you did not specify both “if” and “only if.” Marks
off as well if you only said that the tree is open. Unfinished trees are also open.)
b. Truth-functional falsity A sentence, P, is t-f false if and only if either the tree for P is closed or
(equivalently) every possible truth value assignment can be recovered from the completed tree for
~P.
2. Under what conditions is a tree branch open? A tree branch is open if and only if it is not closed,
which is to say, if and only if an atomic sentence, P, and its negation ~P do not occur on the branch. This
could happen because the branch has not been finished yet or it could be the case after all non-literal
sentences on the branch have been checked off, in which case the branch is not only open but completed.2%
3. Give the first three lines of the tree used to test “A ⊃ B” and “~B ⊃ ~A” for t-f equivalen3%.
Line 1: ~[(A ⊃ B) ≡ (~B ⊃ ~A)]
Line 2 left: A ⊃ B Line 2 right: ~(A ⊃ B)
Line 3 left ~(~B ⊃ ~A) Line 3 right ~B ⊃ ~A
(Points off if the punctuation is not absolutely accurate, esp wrt relative position of “~” and punctuation
marks. Points off as well if the sentences are left-right reversed.
4. Use the truth-tree method to answer a and b below. In each case state your result. Where appropriate,
recover a truth-value assignment that supports your answer. 6%
a. Is the following argument truth-functionally valid?
[C ⊃ (A & U)] ≡ (C v A)
~(A & ~B)
(B & A) ⊃ (~C v ∼U)
C v B Since the tree for the set of the premises plus the negation of the conclusion is closed, the argument
is t-f valid. (Points off for failing to state this.)
b. Are the following sentences truth-functionally equivalent?
A ⊃ (B ⊃ C) (A ⊃ B) ⊃ C
Since the tree for the negation of the biconditional of the two sentences has at least one completed open
branch, the sentences are not t-f equivalent. The truth value assignments recoverable from the leftmost
open branch that prove this are:
A=F, B=T, C=F and A=F, B=F, C=F. (Points off for failing to include this information. Note that you
need not complete the tree if you can find a completed open branch, but that you must do everything on
other branches that is required by a rule application to the branch you are working on. E.g.,
decomposing the sentence at line 2 requires decomposing the sentence on every branch that opens
beneath it, not just the ones on the left that you are working on. No sentences are decomposed on the
rightmost branch because no rule application requires it. Note also that you need only recover
interpretations from one open branch, though you should recover all the interpretations from this branch
that come off of it.) 5. Suppose that the completed truth-tree for a sentence P has at least one open branch and that the
completed truth-tree for a sentence Q has at least one open branch. Does it follow that the tree for P
& Q must have at least one open branch? No. Explain, using an example if appropriate. Consider
the tree for “A & ~A.” This tree closes after one step, applying &D. But the (one line long) trees for
both “A” and “~A” each have a completed open branch.4%
6. Define 6%
a. Derivability (of a sentence from a set of assumptions) in SD. A sentence, P, is derivable
in SD from a set of assumptions, Γ, if and only if there is a derivation in SD that contains
sentences

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