Logic and Reason - Practice
I. Deductive Reasoning
Are the following arguments valid or invalid, true or untrue?
All Italians eat spaghetti
Giovanni Rossi eats spaghetti
Therefore Giovanni Rossi is an Italian
No Martians have red noses.
Rudolph has a red nose.
Therefore Rudolph is not a Martian
Some monks are Tibetans.
All Tibetans are good at yoga.
Therefore some monks are good at yoga.
Some astrologers are frauds.
Some frauds are not wealthy
Therefore some astrologers are not wealthy.
II. What are the missing premises?
Jenny goes to Oxford University, so she must be very intelligent.
Drugs should be legalized because they only harm the addict.
Since it is natural to eat meat, there is nothing morally wrong with it.
III. Suppose Epimenides of crete says to you “Cretans” are always liars.” Is
that true or false?
IV. Evaluating Inferences
Refute the following hypothesis by finding logically possible scenarios that
constitute counter examples to the claims:
1. In order to be completely happy it is necessary to have shelter and some
clothing.
2. It is a necessary condition for being a person to be capable of self-motivated
activities.
3. Every great nation must defend its interests with brute force.
4. If people lose their fear of hell, the will cease to go to church.
Logic and Reason - Practice
5. No rational person would choose to have an abortion if they knew that a fetus
has a soul right from the time of conception.
6. We should always do what makes the majority of people happy.
7. Being honest is the best policy.
8. It is impossible to survive the death of one’s physical body.
9. Every person pursues those things that bring him or her pleasure.
10. It is not possible to fool all people all the time.
In each passage, find the Inference and determine if it is valid.
Passage 1: A group of reputable European scientists has reported that studies
they have conducted do not demonstrate that there is a significant risk of lung
cancer from passive smoking. But this analysis was commissioned by the tobacco
industry. So findings of the analysis are likely to be incorrect.
Passage 2: If it was true that animals have benefited from toxicity tests on
animals more than the test animals have suffered, these tests might be justified.
Unfortunately for those who countenance such tests, the benefit to animals cannot
be established, so toxicity testing on animals is not justified.
Passage 3: If the world’s climate was getting warmer, we would find that some of
the ice at both the North and the South Pole was melting at an unusually high rate.
If the ice was melting, we would see its effect in the raising of the sea level. There
is evidence that this level is increasing, so the world’s climate must be getting
warmer.
Passage 4: The police force should ban their officers from driving at high speed in
pursuit of young joyriders who steal cars. Many deaths, both of joyriders and of
innocent bystanders, have occurred as a result of such chases. The police say that
they have policies which are aimed at preventing danger to the public during car
chases, by requiring police drivers to abandon the chase when speeds become too
high for safety. But the excitement of the chase inevitably makes the police
drivers forget the policy, and disregard public safety. No stolen car is worth a
human life.
Passage 5: Although not all single –parent families are the result of divorce, in the
UK almost ½ of families with dependent children are headed by a single parent.
This compares with an average figure of only 14% in the rest of the EU. These
figures show that divorce has been made far too easy in this country.