# DEDUCTIVE VALIDITY

DEDUCTIVE VALIDITY

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ATS2946 Critical Thinking Assessment Task 2

General directions • This assessment task consists of five questions and is worth 15% of the final

• Word limit: Approximately 300 words.

Deductive validity For each of the following short arguments; (a) say whether the argument is deductively

valid or invalid. Then (b) if you think the argument is invalid, explain why the argument

is invalid – that is, explain how it is possible for the premises to all be true and the

conclusion false. If you think the argument is valid, then represent the logical form of

the argument, making sure you indicate what your schematic letters (A, B etc) stand for.

10 marks for each question, for a total of 50 marks

Example 1 If there are cigarette stubs in the ashtray, Sarah is annoyed. There are no cigarette stubs in the ashtray. Therefore, Sarah is not annoyed. Solution (a) The argument is INVALID. (b) The first premise does not say that cigarette stubs in the ashtray are the only thing that makes Sarah annoyed. So even if there are no cigarettes stubs in the ashtray, Sarah might still be annoyed for some other reason. Example 2 All dogs are mammals and all mammals are carnivores. Therefore, all dogs are carnivores. Solution (a) The argument is VALID. (b) The argument has the following valid form:

1. All A then B 2. All B are C Therefore: C. All A are C A = dogs B = mammals C = carnivores

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Marking scheme

Argument correctly identified as valid or invalid

/4

Explanation for why the argument is invalid OR Valid argument form correctly represented

/6

TOTAL /10

(1) The extinction of the dinosaurs 65 million years ago was due either to a meteorite

impact or to massive volcanic eruptions. Since the meteorite impact theory can now be

ruled out, it must have been the volcanic eruptions that caused the dinosaurs to die out.

(2) All famous movie stars are wealthy. Rose Middleton is wealthy, therefore Rose

Middleton is a famous movie star.

(3) If killing a human being is always morally wrong, then the death penalty cannot be

justified. But there are some situations, such as self-defence, in which it would not be

wrong to kill a human being. So the death penalty is justified.

(4) The potato salad will turn out well if you do exactly what my grandmother’s recipe

tells you to do. The potato salad did not turn out well. So you didn’t follow my

grandmother’s recipe exactly.

(5) People will not participate fully in the political life of their country unless voting is

compulsory. Since voting is not compulsory in the UK, the citizens of that country will

not fully participate in the political life of their country.

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