Often, especially when teaching students who are new to philosophy, it is worth pointing out particular reasoning tactics. The students are very likely to come across similar types of arguments in their readings and working through these exercises may help them understand and critically assess these arguments in future.

What is it in a nut-shell?

The exercise demonstrates a type of reasoning.



Consider the following examples of arguments. The arguments are all fallacious (i.e. mistaken, wrong) in one way or another. Can you identify the mistake?

Two guys in a bar

“Two guys are sitting in a bar.

‘What do you do?’ asks the first.

‘I’m a logician,’ answers the second.

‘What’s that?’

‘It’s someone who does logic.’

‘What is logic?’

‘Well, it’s reasoning that gets you to conclusions. Look, I’ll give you an example. Do you have an aquarium?’

‘Yes, as a matter of fact I do.’

‘So then you must like fish and water.’

‘Well, yes, I do.’

‘So you must like the beach.’

‘Yep, I like the beach.’

‘You take walks on the beach with your girlfriend.’

‘Yeah, I do that.’

‘So you must be heterosexual. That’s logic!’

After the logician leaves the bar, the first guy starts a conversation with someone else. ‘That guy I was just talking to – he’s a logician.’

‘What’s that?’

‘It’s someone who does logic.’

‘What’s logic?’

‘It’s reasoning that gets you to conclusions. I’ll give you an example. Do you have an aquarium?’


‘So you must be a homosexual.’”

From Martin R.M., There Are Two Errors in the the Title of This Book (Canada: Broadview Press, 2004), p.69


It is clear that UFOs (unidentified flying objects) exist, since all the governments of the entire world deny their existence. What else would you expect from a cover-up? 

President Bush Quotes

"Those who enter the country illegally violate the law.", George W. Bush, Tucson, Ariz., Nov. 28, 2005.

"It's in our country's interests to find those who would do harm to us and get them out of harm's way." —George W. Bush, Washington, D.C., April 28, 2005.

"Security is the essential roadblock to achieving the road map to peace." —George W. Bush, Washington, D.C., July 25, 2003

Political Humor.

Logic: this joke works because of the fallacious reasoning. The logician arrives at his conclusion using inference and each step he takes in his reasoning is valid. The interlocutor simply just to the conclusion and funnily enough it is the wrong one.

UFOs: well the governments would also deny the existence of UFOs if indeed there were no such things! This claim is unfalsifiable, i.e. nothing can ever count as evidence against it.

President Bush: The first one is a tautology; i.e. a statement necessarily true, but which gives us no new information as ‘illegal’ means ‘violates the law’ and vice versa. In this instance it is a pretty meaningless statement to make. 

The second one is a muddle over words. Some words have more than one meaning. An argument equivocates if it moves from using a word in one sense to using it in another. For example: I go swimming at the bank The bank is in the city centre Conclusion: I go swimming at the city centre In the first premise ‘bank’ refers to the bank of the river, in the second premise to the financial institution. Bush is mixing up the term ‘do harm to us’ with the term ‘out of harm’s way’.

The third one is so garbled it is difficult to see what it might mean, but it looks like an analogy gone wrong. If peace requires a road map (i.e. a plan) and security is a road block then security is a bad thing as it blocks the road, whereas one would expect the analogy to show how security is pre-requisite or acts in support of peace to make Bush’s point.



To start off, go through this short introduction with all of your students. You should work through this introduction if you are intending to continue with any of the sub-exercises in this section.

Arguments which use analogies have the following form:

Suppose you want to compare X and Y. You know that X and Y are similar in respects a, b and c and you argue that they are therefore probably similar in respects m, n and o. M, n and o are possessed by X but you haven’t observed them yet in Y, you surmise their presence in Y because of the analogy.  

Here’s an example: 

1. I enjoy Greek cuisine.

2. Portuguese cuisine uses the same ingredients as Greek cuisine.

C Therefore I am going to enjoy Portuguese cuisine.

It is important to notice that arguments from analogy are never conclusive and therefore may not be very good proof for what you are trying to argue for. In the example above it may turn out that although Portuguese cuisine uses the same ingredients as Greek, it cooks them in all sorts of different ways and I don’t like the result at all. Thus although Greek and Portugese cuisines are similar in that they use the same ingredients, they are dissimilar in a very important way, i.e. they combine them differently and this leads me to like one cuisine but dislike the other. So X and Y were similar in some respects, but at the same time they were dissimilar in other relevant respects. Other times it may turn out that the initial claim that X and Y are similar is false. 

1. The wolf and the hyena are similar in that they are both pack animals.

2. We have domesticated the wolf (present day dogs).

C Therefore, we could domesticate the hyena.

 According to some scientists the wolf is not similar to the hyena. Wolf packs are hierarchically ordered which makes them more conducive to domestication (as we take on the role of leader of the pack). Hyenas operate as independent individuals and do not share the characteristics that led to the domestication of the wolf. 

Then again, some arguments from analogy can succeed: 

 1. I get travel sick in cars.

2. I get travel sick in trains.

C Therefore I will get travel sick in airplanes. 

And I can come to this conclusion without having ever been in an airplane. Can you come up with an argument from analogy (whether successful or disputed)?

Now split your students into small groups and ask them to:

Now consider the following arguments from analogy used by philosophers to make their points and see whether they are successful:

Lifeboats and the Poor 

Some of the world’s inhabitants have more money and resources than others. Those who have less, often have very little and in some parts of the world people suffer extreme poverty (where this is taken to mean having insufficient funds for food, clothes, shelter, clean water and basic medical care). Given this disparity in wealth what are the obligations of the rich to the poor?

Philosopher Peter Singer presents (and then goes on to argue against) an argument by Garrett Hardin based on an analogy:

“In support of this view Garrett Hardin has offered a metaphor: we in the rich nations are like the occupants of a crowded lifeboat adrift in a sea full of drowning people. If we try to save the drowning by brining them aboard our boat will be overloaded and we shall all drown. Since it is better that some survive than none, according to Hardin, ‘lifeboat ethics’ apply. The rich should leave the poor to starve, for otherwise the poor will drag the rich down with them.” Singer P., Practical Ethics (Cambridge: Cambridge University Press, 1979), p.175.

First of all see if you can make sense of Hardin’s analogy:

  • What are the two things he is trying to compare (the X and Y of the analogy)?
  • In what respect are these two things similar (i.e. the observed similarities, like the a, b and c similarities of the analogy)?
  • Therefore, in what other respects are they also similar (i.e. the conclusion of the analogy, the m, n and o kind of similarities)?

Can you see any possible reason why Hardin’s argument from analogy might not work? 

Hardin is comparing the current situation between the rich and the poor in the world with a crowded lifeboat adrift in the sea. They are similar in that in both cases one group of people is safe and well (the rich and the people in the lifeboat) and another group of people are in danger (the poor and the ones drowning). If the lifeboat occupants were to rescue all the drowning everyone would die because the lifeboat is too small for everyone. Similarly if the rich were to try to save all the (future) poor by sharing ressources everyone would die.

If you wanted to attack Hardin’s argument from analogy how would you go about it? How many different criticisms can you come up with? How do they relate to how arguments by analogy work in general?

Allow your students a few minutes to work on this on their own and see what ideas they come up with. If they are having difficulty coming up with arguments (which are relevant to the present discussion about the use of analogies in arguing for this topic) then ask them to read further. If they are doing well, then continue discussing their ideas.


Suppose someone made the following points in response to Hardin, how would this affect his argument? What happens to the argument from analogy in each case?

“I completely disagree with Hardin. The poor would not overwhelm the lifeboat and in fact there is plenty of room for them. The rich are rich enough to support very large numbers of poor people. It may be that as health care levels improve there may be a temporary increase in the population, but as education is more widely available and women are emancipated, there will be more contraceptive control and the population will decline overall. If anything there will be fewer people overall than Hardin predicts and they will be better able to look after themselves if we help them now.”

“Well that would all be true but for one fact: the rich are not just rich, they are really rich. The rich spend their money on luxuries and things they could easily do without. If they helped the poor, they would not be depriving themselves of essential goods, but of extravagant luxuries they can easily do without.”

“A am an old man and have lived a long and happy life. If I was occupying a seat in the lifeboat that could go to another person, I would give it up.”

This argument changes the characterization of the current situation with respect to the poor. The poor are not analogous to people drowning by an overcrowded lifeboat, but to people drowning by a half-empty lifeboat. Given that we’d have to allow people on board our half-empty lifeboat, we have to help the poor. We now have an argument from analogy that differs from the original and makes the opposite point.

Another suggestion is to refuse to accept the similarities. The current situation between the rich and the poor is nothing like a lifeboat situation. The rich have enough resources to help the poor without risking their own lives. The rich are not barely surviving by clinging to a lifeboat, they are, if you want to carry the analogy further, they are taking up extra seats with their luxurious habits. All they need to do to save the poor is accept a reduction in their standard of living, so that everyone can fit on basic boats.

This one is rather extreme, but people may think we have an obligation to give up our own seats to those who are younger, or more useful than us, to our family or our friends. This line of attack questions Hardin’s assumption that the ones in the lifeboat would hold onto their seats no matter what.

The Gift of Life

Consider the following argument against suicide based on an analogy: “God bestows upon us the gift of life. To commit suicide, to take one’s own life, is to show ingratitude and disrespect to God for his gift.”

  • What is being compared to what?
  • What is wrong with giving away a gift in general?
  • In what respects is a life like a gift?
  • What is wrong with giving away a life?
  • Does this analogy work?

This does not seem like a very strong argument. The analogy compares life to a gift. The implicit claim is that it is wrong to give away a gift. Now when gifted the gift becomes the property of the recipient and although it may be insensitive to give away a gift, the whole point of something being a gift is that it is given away for the recipient to do with as he pleases. So if life is like a gift then that in itself does not imply that we cannot choose to reject it.

Hacking and Chess
(I am grateful to Rob Lawlor for sharing this analogy)

Consider the following argument in support of hacking based on analogy: “Hacking, should not be perceived in derogatory terms, rather it is a beneficial exercise of the intellect rather like chess. It is intellectually demanding, it involves planning, strategy and the ability to outwit your opponent.”
  • What is being compared to what here?
  • Why is chess a beneficial and complimentary activity?
  • In what respects is hacking like chess?
  • Does this analogy work?

Again this is a rather weak argument. Chess and hacking may share some characteristics, such as being intellectually demanding, but hacking seems to have some other qualities which separate it from innocuous pastimes such as chess. Hacking involves the unauthorized violation of computer systems and can be done with the intent to steal data or cause damage. This aspect of hacking means that the analogy breaks down as the two practices are now significantly different.

Same-Sex Marriages 

Consider the following argument is support of same-sex marriages using analogy: “Heterosexual couples have a legal right to marry and have their union enjoy the protection of law (e.g. through tax breaks, through the recognition of the spouse as the appropriate decision maker in medical emergencies, through the acceptance of joint custody over their children). The right to marry is based on the claim that a person’s choice of sexual partner is fundamental to her well-being and protected under a right to privacy, a right to equal treatment under the law and a right to found a family.”

  • What is being compared to what here?
  • What is the basis for a right to marry for heterosexual couples?
  • In what respect are same-sex couples similar to heterosexual couples?
  • Does this analogy work?

This one may prove to be more controversial as there may be some discussion about whether same-sex couples are similar in all relevant respects to heterosexual couples. The argument suggests that they are, since they share all relevant claims, i.e. claim to privacy, to equal protection under the law, to starting a family. The one difference, i.e. one couple is same sex, one is different sex, is a difference but it is not relevant for the present purposes. Therefore, the benefits that are due to heterosexual couples are also due to same-sex couples. To disprove this argument one would have to show that the difference is significant in such a way that same-sex couples do not have the same entitlements as heterosexual couples.



Start off by splitting your students into small groups and asking them to consider the first claim:

Consider the following claim: “Abbey was ill with cancer. She visited a psychic healer who manipulated her aura to remove the evil energy causing the cancer. Abbey was cured because she visited a psychic healer.”

Do you agree with the conclusion, i.e. that Abbey was cured because she visited the psychic healer?

What possible problem could there be with this kind of reasoning?

This is a problem with causation. Abbey was indeed healed but it is a mistake in reasoning to assume that this was brought about by the psychic healer. For one, the example does not specify whether Abbey was on any conventional medication. If she was, then this could be the cause of her recovery not the healer. Furthermore, spontaneous remission is also a possibility, there are many things we don’t yet understand about cancer so perhaps something in the development of the disease in this case was favorable for this patient. The conclusion here is that just because one event follows another (recovery followed the visit to the psychic healer) does not mean that the first event caused the second one. It is important that your students understand this general point and how it can affect the validity of an argument. Now ask them if they can come up of a similar mistake in reasoning. They should be able to come up with all sorts of examples, for example, “The gods had to be appeased with a human sacrifice before they allowed the sun to appear again” is confused about the causes of a sun eclipse. Now split your students into small groups. Ask each group to consider one case below and report back to the wider group on what might be wrong with the reasoning in each example. Ask them to try to come up with a general conclusion about what has gone wrong in each case as we did in the case above. You may need to help your students see the problem in each case by working with them in their small groups:

Group A: “Studies show that children who play violent computer games are prone to violent behavior. Therefore playing violent computer games makes children prone to violent behavior.”

Here the order of causation is ambiguous. It may be that playing violent computer games causes children to be prone to violence, but it could also be that children who are prone to violence anyway are also attracted to violent computer games. The claim does not explain why the first conclusion is preferred rather than the second and this is a good demonstration of how statistical evidence can be interpreted in a variety of ways. The conclusion here is that just because we observe a connection between A and B it is not always possible to decide without further evidence whether A caused B, or B caused A.

Group B: “Drinking a lot of coffee can help cancel out the detrimental effects on the liver of excessive alcohol consumption. Therefore, drinking a lot of coffee should be part of a healthy lifestyle.”

The first part of the sentence is true, there is indeed some evidence to show that coffee drinkers have a lower risk of alcohol-related liver diseases (see for example Science Daily, “Coffee Drinking Associated with Lower Risk of Alcohol Related Liver Disease”), so this causal link is correct. However, the second causal link, between coffee and health, required to establish the conclusion is not valid. A healthy lifestyle does not involve drinking excessive amounts of alcohol (of the kind that would bring on liver disease) and therefore there would be no need to drink the coffee to cancel the excessive alcohol drinking. A healthy lifestyle requires, contra the conclusion, abstaining from (excessive?) coffee and alcohol drinking. The conclusion here is that we should be careful of the terms under which a conclusion is arrived at, i.e. if you drink a lot of alcohol, it is then healthier to drink a lot of coffee as well, but this is conditional on drinking the alcohol in the first place.

Group C: “Unlike the US, the UK provides free heroin to drug users. However, the UK has not seen a decrease in drug use so there is no reason for the US to adopt the same policy.”

Now this case is mixing up two different arguments. Drug users commit serious crime in their pursuit of drugs. When these drugs are freely available as in the UK there is a corresponding drop in crime. There is a causal connection between free drugs and crime reduction. This is true in the UK and could thus, arguably, be true in the US were the US to adopt a similar policy. However, there is no causal connection between the provision of free drugs and a reduction in the numbers of drug users, neither in the UK or the US. The conclusion here is that we need to know what effects a certain cause can have, i.e. free drugs lead to a reduction in crime not a reduction in drug use.

Group D: “Arranged marriages are the best way to ensure happy and successful relationships. You only have to look at the incidence of divorce to see how many people are unhappily married.”

This case suggests a correlation between arranged marriages and successful marriages but doesn’t really provide any evidence for this at all. It is true that divorce rates have been on the increase since divorce first became an option. It could also be true that there is a link between the divorce rate and unhappy marriages, but none of that links arranged marriages with happy marriages. This example relies on totally unrelated claims and assumes a causal relationship between them. The conclusion is that sometimes there is no causal link whatsoever between the statements on which a particular conclusion relies on.