Flaws in reasoning

Some common flaws in reasoning! Need to know for exam!

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Ad Hominem

Meaning 'to the man', it refers to dimsissing someone's argument becuase of some characteristic of the arguer. The honest way to challenge an argument is to examine and point out faults in logic or evidence, but the critic guilty of this flaw fails to look at the content of the argument and instead disparages the arguer for some quality that is unrelated.


Bob Geldof called his daughters ridiculous names such as Fifi Trixibelle. He's evidently a man who cannot be taken seriously so there is no point in listeing to his arguments about how to address African Poverty.

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Correlation equals cause confusion

Many arguments rest on the assumption that if two factors are found to correlate, one has to be caused by another.


If extending the licensinn hours was followed by a swift increase in adult obesity, many people might assume that the first had caused the second (as people put on weight by drinking more), whereas in fact they may be unrelated. Many changes occur in society during any period of time, so to isolate just two and argue that one causes the other is poor reasoning

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Circular Argument

This argument appears ar first to be based on a reason, but, on careful examination, the reason rests on evidence that can only be believed if the conclusion is accepted


'I believe in God because the Bible says that He exists'

'How do you know the Bible is telling the truth?'

'Of course the Bible is true. It's the Word of God'

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Conflation refers to the error of treating two concepts, e.g. obesity and unfitness, as if they were exaclty the same. The arguer might be making a fairly convincing case in relation to the first concept but then change the terms in a way that might easily escape notice. The weakens the argument.


Infants have no life experience but a great deal of curiosity, so they often investigate items such as electric sockets with no awareness of danger. It is the duty of every parent to safegaurd those in their care. Therefore, parents who leave children unsupervised, even for a quick visit next door, should be prosecuted.

The change of term from infants to children has weakened this argument.


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Generalisation, sweeping, unwarrented or hasty

This is an unwarrented universal claim, such as the statement

Males are good at science

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Necessary or sufficent conditions confused

A necessary condition is one that is needed in order for something to happen:


Grade A in physics being required for an engineering course.

A sufficent condition is one that guarantees that the next step must follow:


Grade A in physics would not be enough to get into the university course. You also need a good personal statement and references.


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Post hoc

This flaw involves the belief that a phenomenon is caused by something preceding it, based on no other evidence than order of events.


If someone gives you a 'lucky mascot' to take into an exam and you obtain an exeptionally good grade, you may be led to believe that the mascot caused your grade, and not your hard work and revision.

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Predictions about the future without sufficent evi

A common example of this flaw is to project trends such as recent increases in crime or divorce into the future, assuming that nothing will occur to change the patterns, resulting in an alarming prospect.


As a large proportion of the population is obese now than 20 years ago, a commentator might predict the same rate of increase in the next 20 years and the same in the following 20.

However, experience tells us that there is likely to be a change in the pattern.

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Restricting the options

This type of argument describes a limited number of possibilities as if they were the only ones, persuading listeners to choose the least unattractive option when in fact there could be unmentioned ones that they might prefer.


The Conservative Party hasn't come up with convincing enough policies so you ought to vote for Labour


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Slippery slope or thin edge of the wedge

The slippery slope usually involves warning about the disaster to which a particular course of action or trend could lead. It uses a chain of reasoning to build up a step by step prediction, describing how things could go from bad to worse. The flaw lies with weak links in the chain, as insufficent evidence is provided at various points to prove that dire events predicted will actually take place.


Parents anxious about the safety of their children should be discouraged from buying GPS tracking devices. They will be tempted to check up on their youngster's whereabouts unecessarily often and this will irritate the children. Children who feel their independence is threatened will rebel against their parents by adopting patterns of behaviour that they associate with adulthood. This will tempt them into smoking, drinking alcohol, and precocious sexual acitivity. Lung disease, liver damage, early pregnancies and STD's will result, leading to premature death. Therefore, parents concerned about their children's safety are simply signing their death warrants by ordering GSP tracking devices. 

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Straw man

Also known as 'straw person', this flaw in argument involves the arguer identifying some minor weakness in an opponent's argument or in some scheme or policy. This weakness is exaggerated and used as a means to reject the whole argument or scheme. Schemes may have a minor weakness but be viable because they have multiple advantages.

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Tu quoque

The tu qouque rejoinder involves deflecting what might be sound criticism by accusing the critic of being guilty of the same or a similar fault.


If a student is reprimanded for being late for a lesson argued back that the teacher they had been late for a previous lesson does not justify the student's lateness.

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