Learning Outcomes
- Discern between an inductive argument and a deductive argument
- Evaluate deductive arguments
- Analyze arguments with Venn diagrams and truth tables
- Use logical inference to infer whether a statement is true
- Identify logical fallacies in common language including appeal to ignorance, appeal to authority, appeal to consequence, false dilemma, circular reasoning, post hoc, correlation implies causation, and straw man arguments
In the previous discussion, we saw that logical arguments can be invalid when the premises are not true, when the premises are not sufficient to guarantee the conclusion, or when there are invalid chains in logic. There are a number of other ways in which arguments can be invalid, a sampling of which are given here.
Ad hominem
An ad hominem argument attacks the person making the argument, ignoring the argument itself.
Example
“Jane says that whales aren’t fish, but she’s only in the second grade, so she can’t be right.”
Example
“Jane says that whales aren’t fish, but everyone knows that they’re really mammals—she’s so stupid.”
Try It
Appeal to ignorance
This type of argument assumes something is true because it hasn’t been proven false.
Example
“Nobody has proven that photo isn’t Bigfoot, so it must be Bigfoot.”
Appeal to authority
These arguments attempt to use the authority of a person to prove a claim. While often authority can provide strength to an argument, problems can occur when the person’s opinion is not shared by other experts, or when the authority is irrelevant to the claim.
Try It
Example
“A diet high in bacon can be healthy – Doctor Atkins said so.”
Example
“Jennifer Hudson lost weight with Weight Watchers, so their program must work.”
Appeal to Consequence
An appeal to consequence concludes that a premise is true or false based on whether the consequences are desirable or not.
Example
“Humans will travel faster than light: faster-than-light travel would be beneficial for space travel.”
Try It
False dilemma
A false dilemma argument falsely frames an argument as an “either or” choice, without allowing for additional options.
Example
“Either those lights in the sky were an airplane or aliens. There are no airplanes scheduled for tonight, so it must be aliens.”
This argument ignores the possibility that the lights could be something other than an airplane or aliens.
Try It
Circular reasoning
Circular reasoning is an argument that relies on the conclusion being true for the premise to be true.
Example
“I shouldn’t have gotten a C in that class; I’m an A student!”
In this argument, the student is claiming that because they’re an A student, though shouldn’t have gotten a C. But because they got a C, they’re not an A student.
Try It
Straw man
A straw man argument involves misrepresenting the argument in a less favorable way to make it easier to attack.
Example
“Senator Jones has proposed reducing military funding by 10%. Apparently he wants to leave us defenseless against attacks by terrorists”
Here the arguer has represented a 10% funding cut as equivalent to leaving us defenseless, making it easier to attack.
Post hoc (post hoc ergo propter hoc)
A post hoc argument claims that because two things happened sequentially, then the first must have caused the second.
Example
“Today I wore a red shirt, and my football team won! I need to wear a red shirt everytime they play to make sure they keep winning.”
Try It
Correlation implies causation
Similar to post hoc, but without the requirement of sequence, this fallacy assumes that just because two things are related one must have caused the other. Often there is a third variable not considered.
Example
“Months with high ice cream sales also have a high rate of deaths by drowning. Therefore ice cream must be causing people to drown.”
This argument is implying a causal relation, when really both are more likely dependent on the weather; that ice cream and drowning are both more likely during warm summer months.
Try It
Try It
Identify the logical fallacy in each of the arguments
- Only an untrustworthy person would run for office. The fact that politicians are untrustworthy is proof of this.
- Since the 1950s, both the atmospheric carbon dioxide level and obesity levels have increased sharply. Hence, atmospheric carbon dioxide causes obesity.
- The oven was working fine until you started using it, so you must have broken it.
- You can’t give me a D in the class—I can’t afford to retake it.
- The senator wants to increase support for food stamps. He wants to take the taxpayers’ hard-earned money and give it away to lazy people. This isn’t fair so we shouldn’t do it.
