The process we use to reflect on, assess and judge the
assumptions underlying our own and others ideas
• The Argument
• The Conclusion
• The Evidence
• The Assumption
• The Inference
• Fallacies in logic
MAIN NOTIONS OF CRITICAL REASONING
MAIN NOTIONS OF CRITICAL REASONING
since because for
hence so thus
IDENTIFY EVIDENCEAND CONCLUSION
1. Mary has excellent marks at school. Therefore, she has good
chances to enter a university.
2. The new manufacturing process must be more efficient than the
previous one since the company profits have increased significantly.
3. Time management has always been crucial to top-level managers.
According to the recent survey, 9 out of 10 top executive managers
attribute their success to their timing strategies.
4. The recent terroristic attack in Livonia has showed that terrorists can
hack virtually any computer protection system. We should protect our
borders more thoroughly.
5. Photography is no longer an art form. Nowadays everyone has
access to digital cameras that only need to be pointed at the subject
in order to generate a perfect image.
So, what is an ASSUMPTION?
… or rather where is it?
Assumption is an underlying premise that invalidates
the conclusion of the argument. It is an obligatory
condition that must be thought of by the speaker before
he reaches conclusion.
1. assumption is NOT stated within the argument
2. Evidence + Assumption = Conclusion
3. If assumption is negated, conclusion falls apart
4. Assumption is a secondary reason for conclusion
5. Assumption is never a restatement
TRY TO COME UP WITH YOUR
1. The Apple company has no major competitors but Samsung.
Interestingly, Apple exports 72% of all its cell phones annually. In
contrast, the share of Samsung cell phones that are sold in export
market barely reaches 52%. This stands to prove that the number of
Apple phones bought by foreign retailers is bigger than that of
2. Because no employee wants to be associated with bad news in the
eyes of a superior, information about serious problems at lower levels
is progressively softened and distorted as it goes up each step in the
management hierarchy. The chief executive is, therefore, less well-
informed about problems at lower levels than are his or her
subordinates at those levels.
3. On Monday, John, who had the flu at the time, took Marissa out to
the cinema. In two days, Marissa showed signs of flu. So, if Marissa
had not agreed to go to the movie, she would not have become ill.
1. What is an argument?
2. What constitutes an argument?
3. Name a synonym for ‘underlying premise’.
4. Where are we supposed to look for assumption?
5. What is the most peculiar feature of an
1. Bacteria from food can survive for several days on the surface of plastic cutting
boards, but bacteria can penetrate wooden cutting boards almost immediate in
leaving the surface free of contamination. Therefore, wooden cutting boards, unlike
plastic cutting boards, need not be washed in order to prevent their contaminating by
food that is cut on them; wiping them off to remove food debris is sufficient.
Which one of the following is an assumption on which the argument depends?
(A) Washing plastic cutting boards does not remove all bacteria from the surface.
(B) Prevention of bacteria contamination is the only respect in which wooden cutting
boards are superior to plastic cutting boards.
(C) Food that is not already contaminated with bacteria can be contaminated only by
being cut on contaminated cutting boards.
(D) Bacteria that penetrate into wooden cutting boards do not reemerge on the
surface after the cutting boards have been used.
(E) Washing wooden cutting boards kills bacteria below the surface of the cutting
2. A factory in China has two options to improve efficiency: adding robotic assembly
lines and subcontracting out certain small production goals that could be done more
efficiently elsewhere. Adding robotic assembly lines will improve efficiency more than
subcontracting some small production goals. Therefore, by adding robotic assembly
lines, the factory will be doing the most that can be done to improve efficiency.
Which of the following is an assumption on which the argument depends?
(A) Adding robotic assembly lines will be more expensive than subcontracting some
small production goals.
(B) The factory has a choice of robotic assembly lines, some of which might be better
suited to this factory than others.
(C) The factory may or may not decide to choose either alternative.
(D) Efficiency cannot be improved more by using both methods together than by
adding robotic assembly lines alone.
(E) This particular factory is already the third most efficient factory in China.
Neurobiologist Sapolsky from Stanford University used a blowgun to tranquilize
baboons in the Kenyan savanna. Once immobile, he could attach radio collars and
track the baboon's movement. In one study, whenever a baboon's radio collar slipped
off he would put it back on. This would involve another session of immobilizing the
baboon with a tranquilizing dart. He noticed that female baboons that were frequently
recollared had significantly lower fertility rates than uncollared females. Probably,
therefore, some substance in the tranquilizer inhibited fertility.
Which of the following is an assumption on which the argument depends?
(A)The dose of tranquilizer delivered by a tranquilizer dart was large enough to give the
Sapolsky a generous margin of safety.
(B)The fertility rate of uncollared female baboons had been increasing in the past few
(C)Any stress that female baboons might have suffered as a result of being immobilized
and handled had little or no negative effect on their fertility.
(D)The male baboons did not lose their collars as often as the female baboons did.
(E)The tranquilizer used in immobilizing baboons was the same as the tranquilizer used
in working with other large mammals.
To infer means to conclude or summarize. An inference is a conclusion deduced from
a set of evidences/facts.
• Inference is something that must be true on the basis of
the given information;
• Mostly, any inference is a sort of restatement;
• To easily identify an inference, you should tackle the
problem this way: [Evidence], hence [Inference];
• Inference does not usually go beyond the story;
• Inference would use vague language.
1. High levels of fertilizer and pesticides, needed when farmers try to produce high
yields of the same crop year after year, pollute water supplies. Experts therefore urge
farmers to diversify their crops and to rotate their plantings yearly. To receive
governmental price-support benefits for a crop, farmers must have produced that
same crop for the past several years.
The statement above, if true, best support which of the following conclusions?
A: The rules for governmental support of farm prices work against efforts to reduce
B: The only solution to the problem of water pollution from fertilizers and pesticides is
to take farmland out of production.
C: Farmers can continue to make a profit by rotating diverse crops, thus reducing
costs for chemicals, but not by planting the same crop each year.
D: New farming techniques will be developed to make it possible for farmers to
reduce the application of fertilizers and pesticides.
E: Governmental price supports for farm products are set at levels that are not high
enough to allow farmers to get out of debt.
2. Some planning committee members—those representing the construction industry
—have significant financial interests in the committee’s decisions. No one who is on
the planning committee lives in the suburbs, although many of them work there.
If the statements above are true, which one of the following must also be true?
(A) No persons with significant financial interests in the planning committee’s
decisions are not in the construction industry.
(B) No person who has significant financial interest in the planning committee’s
decisions lives in the suburbs.
(C) Some persons with significant financial interests in the planning committee’s
decisions work in the suburbs.
(D) Some planning committee members who represent the construction industry do
not work in the suburbs.
(E) Some persons with significant financial interests in the planning committee’s
decisions do not live in the suburbs.
3. Shipping Clerk: The five specially ordered shipments sent out last week were sent
out on Thursday. Last week, all of the shipments that were sent out on Friday
consisted entirely of building supplies, and the shipping department then closed for
the weekend. Four shipments were sent to Truax Construction last week, only three
of which consisted of building supplies.
If the shipping clerk’s statements are true, which of the following must also be true?
A. At least one of the shipments sent to Truax Construction last week was specially
B. At least one of last week’s specially ordered shipments did not consist of building
C. At least one of the shipments sent to Truax Construction was not sent out on
Thursday of last week.
D. At least one of the shipments that were sent out on Friday of last week was sent to
E. At least one of the shipments sent to Truax Construction last week was sent out
FALLACIES IN LOGIC
A logical fallacy is a flaw in the author’s reasoning.
Generally, they distinguish the following logical fallacies:
- Invalid cause-effect connection;
- Insufficient evidence;
- Improper conclusion;
- Inaccurate assumption;
- Other potential factors overlooked by the author.
WHAT IS ILLOGICALABOUT
THE FOLLOWING CASE?
Yesterday, Mary went swimming in a cold sea. Today, she is
suffering from a fever. Mary must have got sick because of
cold sea water.
Homeowners aged 40 to 50 are more likely to purchase ice cream and
are more likely to purchase it in larger amounts than are members of any
other demographic group. The popular belief that teenagers eat more ice
cream than adults must, therefore, be false.
The argument is flawed primarily because the author
(A) fails to distinguish between purchasing and consuming
(B) does not supply information about homeowners in age groups other
than 40 to 50
(C) depends on popular belief rather than on documented research
(D) does not specify the precise amount of ice cream purchased by any
(E) discusses ice cream rather than more nutritious and healthful foods
Countries A and В are in competition to draw tourists to their countries. In
Country A, about 2,500 violent crimes are reported per year. In Country
B, only about 1,000 violent crimes are reported per year. Trying to draw
tourists away from Country A, officials in Country В use these violent
crime statistics to claim it has a lower violent crime rate than Country A.
Which of the following, if true, would expose the flaw in Country B’s
argument that it has the lower violent crime rate?
(A) Most violent criminals in Country В are repeat offenders
(B) White-collar crime is higher in Country В than in Country A.
(C) The population of Country A is twenty times greater than the
population of Country B.
(D) Country В has fewer tourists than Country A.
(E) Country A has a better prison system than Country B.