Formal logic can help us achieve clarity and help us make sure our arguments are relevant in various ways, but there are other requirements for a good argument. Most philosophers seem to get caught up discussing fallacies (errors in reasoning) rather than good reasoning. I will discuss the following four requirements for writing good arguments and the corresponding fallacies for failing to achieve the requirements:
- Supporting Evidence
- Relevant Evidence
- Consider all Viable Options
1. Supporting Evidence
Arguments must have premises and a conclusion. Premises are statements that must be accepted before the conclusion is accepted. Premises could be considered evidence for the conclusion, but even controversial premises be supported by evidence. Sometimes we might accept a premise because it is taken for granted as true, but philosophers often try to support every somewhat controversial premise they can.
An argument can only have relevant supporting evidence if (a) the conclusion is appropriately modest, (b) the conclusion isn’t overly ambitious, (c) the premises are more likely true than the conclusion, and (d) the conclusion isn’t illegitimately absurd.
(a) Modest conclusions
The premises of an argument must lead us to the conclusion. If the argument is valid and the premises are true, then we have no choice but to accept the conclusion. Such a conclusion is appropriately modest. For example:
- If Socrates is a man, then he is mortal.
- He is a man.
- Therefore, he is mortal.
The conclusion must be accepted because the premises are almost certainly true.
(b) Overly ambitious conclusions
A conclusion that is not sufficiently supported by the premises is an inappropriate type of conclusion. Such conclusions tend to be what I call overly ambitious. For example:
- If Socrates is a philosopher, then he is relatively wise.
- Socrates is a philosopher.
- Therefore, Socrates knows that the Earth is the third planet from the sun.
It is common knowledge that the Earth is the third planet from the Sun and we would expect that relatively wise people would know such a thing, but that is only common knowledge in our day and age. Socrates was from a different period of time, so he might have not known. The conclusion is overly ambitious because we can only accept the conclusion given certain unstated assumptions.
(c) Premises that are likely true
The premises should be more likely true than the conclusion. Conclusions must not be more likely true than the premises or the premises aren’t really good evidence for the conclusion. Imagine that someone argues the following:
- If people exist, then atoms exist.
- People exist.
- Therefore, atoms exist.
This argument is logically valid, but we are more certain that people exist than atoms exist. The fact that people exist shouldn’t be accepted as evidence that atoms exist. That isn’t to say that atoms don’t exist. We just know less about atoms existing than people.
To have a conclusion that is more likely true than the premises is a fallacy, but I don’t know the name of it. It might not have a name. I will call such a fallacy an argument with a trivial conclusion because the conclusion is already known to be likely true and the premises do not properly help assure us that the conclusion is true.
(d) Absurd conclusions
Even worse than having a trivial conclusion is having an illegitimately absurd conclusion. Absurd conclusions are extraordinary claims, and my point here is merely that extraordinary conclusions require extraordinary evidence. An absurd conclusion can be a conclusion that is almost certainly false even if the premises seem to be probably true. It is impossible to have an illegitimately absurd conclusion if the conclusion is appropriately modest, so the premises used must be questionable. But the fact that questionable premises can lead us to an absurd conclusion just makes us that much more certain that a premise has to be false. For example:
- I have existed for over twenty years.
- The past resembles the future.
- Therefore, I will always exist.
Some people can’t imagine not being immortal and they believe they probably have an immortal soul. After all, we have existed all our lives. However, the conclusion is so incredible that one of the premises is almost certainly false. In this case the premise “the past resembles the future” seems to be taken the wrong way. If we are ever to accept the immortality of the soul, we will need a much better argument than this.
To have an absurd conclusion, such as the argument above, is to commit a fallacy, but I don’t know if it has a name. I will call it the “absurd conclusion” fallacy. In many cases this fallacy will be a type of suppressed evidence fallacy—there is usually information that will undermine the conclusion that isn’t mentioned. Such information can be more important than the supporting premises for the conclusion.
Some people call the absurd conclusion fallacy a ‘reductio ad absurdum’ or a ‘reductio’ for short. However, there are nonfallacious ways to reason using a reductio ad absurdum.
Moreover, the above argument has insufficient premises to prove the conclusion. Even if the premises are true, the conclusion might not be true.
Another example of an argument with an absurd conclusion is the following:
- Many people have past life experiences that provide insightful information.
- Either past life experiences are caused by past lives or dreams.
- It is very unlikely that a dream would lead to such insightful information.
- Therefore, some people probably have past lives.
Although all of the premises look very likely to be true, the conclusion is absurd. The problem is that we have good reason to suspect that at least one premise is false. Perhaps there are other explanations for past life experiences than the two mentioned, and perhaps dreams can lead to the insightful information given by past life experiences. The fact is that we are uncertain about the premises all being true, and extraordinary conclusions require extraordinary evidence. Such extraordinary evidence isn’t given.
2. Relevant Evidence
The evidence used in arguments must be relevant or we have no reason to trust them. The fact that bread has always been nutritious (rather than poisonous) makes it reasonable to think bread is still nutritious. However, people seem to be drawn to distractions (red herrings) rather than relevant considerations.
For example, the fact that the Soviet Union had universal health care doesn’t prove that universal health care is wrong. If everything the Soviets did was wrong, then we should never eat food or have sex. The temptation to dismiss some idea out of hand just because it was endorsed by an undesirable person or society is just a problem with human psychology.
The fallacy of using irrelevant evidence for a position is often called the red herring fallacy. Red herring fallacies are very common when people give objections to other arguments. Why? We often ignore the other person’s argument and just argue against their conclusion. It is often inappropriate to reject an argument just by rejecting the conclusion. For example, consider the following argument:
- Abortion kills people.
- Killing people should always be illegal.
- Therefore, abortion should be illegal.
A common response is that abortion should not be illegal because that would violate women’s rights. However, women’s rights are somewhat irrelevant to the above argument. If we accept the premises, then the conclusion follows. If women’s rights prove that abortion isn’t forbidden, then we are stuck with contradictory arguments for and against the conclusion: Abortion should be legal and illegal.
A good objection to the abortion argument is likely going to oppose one of the premises. Is killing people always wrong? If so, war should be entirely illegal. Does abortion kill people? Why should we think fetuses are people?
If we find it important to argue against someone’s conclusion, then we can do so, but we should often also give objections against the arguments given in support of the conclusion we reject.
An obvious example of the red herring fallacy is the following:
- Bob argues that “we shouldn’t throw people into prison who are found guilty of illegal drug possession because it makes everyone worse off.”
- However, Bob probably takes marijuana and just doesn’t want to go to prison for it.
- Therefore, we should reject Bob’s argument.
This argument gives us no good reason to reject Bob’s argument. It actually changes the subject, then says we should reject Bob’s argument. Fallacious arguments are generally not written in such an explicit way and the conclusion in particular would probably not be explicitly stated in real life.
3. Consider all Viable Options
Arguments require that we accept that something is true instead of something else. Every single premise could be challenged if it doesn’t describe reality perfectly. When we argue that Einstein’s theory of relativity is true, we need to take a look at how much better it is than any alternative. The same goes for any other theory.
It is difficult for us to consider every viable option, so beliefs that fail to do so are common. Consider the following assertions:
- Either communism is good or universal health care is bad.
- Ether creationism is true or evolution is true.
- Either God exists or morality is just a matter of taste.
We might wonder, Why can’t both be false? Maybe capitalism is mostly good, but universal health care can also be good. There are often more options than we see at first. To assume that there are less viable options than those that really exist is to suppress evidence and commit a popular fallacy known as the false dilemma.
An obvious example of the false dilemma fallacy is the following:
- Either Socrates is a horse or a cow.
- Socrates isn’t a horse.
- Therefore, Socrates is a cow.
The problem is that the first premise is false because it does not lay out the most plausible options. In particular, it should say “Either Socrates is a human, a horse, or a cow.”
When we object to an argument or belief, we need to be able to describe it and fully understand it, or what we have to say against it will be irrelevant. To properly describe an argument or belief is to be charitable. To misrepresent another person’s view destroys our chances of saying anything relevant about it. Consider the following argument:
- People who believe in God live a better life.
- We have some reason to engage in actions that offer us benefits.
- Therefore, we have some reason to believe in God.
Someone might then disagree with you and say the following: “You are saying that everyone has to believe in God because it offers us benefits, but we shouldn’t have to believe something just because it benefits us in various ways!” This objection has misrepresented the argument because having some reason to believe in God is much different from the belief that “everyone must believe in God.” Such an objection is irrelevant.
To misrepresent other people’s arguments by making their argument worse is a fallacy called the straw man. However, to misrepresent someone’s argument by improving it isn’t wrong. Rather than argue against a flawed argument, we should generally want to argue against the best argument we can.
Supporting evidence, relevant evidence, considering all viable options, and charity are essential for any good debate. People often fail to live up to these standards, and such standards are difficult to live up to without experience. People with no experience with logic have an even greater chance of providing us with inappropriate supporting evidence (trivial conclusions), overly ambitious conclusions, absurd conclusions, red herrings, false dilemmas, and straw men.
For more information on fallacies, you might like this website: