## 1.5 Bad Arguments — Logical Fallacies

Hui: 这本短小的书对常见的逻辑谬误（Art of Thinking Clearly）进行了清晰直观的解释，比如straw man fallacy，the slippery slope argument，ad hominem attack等。

• Argument from Consequences: speaking for or against the truth of a statement by appealing to the consequences it would have if true (or false)

• If God does not exist, then everything is permitted. (Dostoevsky)

• Straw Man: intentionally caricature a person’s argument with the aim of attaching the caricature rather than the actual argument.

• My opponent is trying to convince you that we evolved from chimpanzees who were swinging from trees, a truly ludicrous claim

• Appeal to Irrelevant Authority: appeal to the feeling that others are more knowledgeable

• Appeal to ancient wisdom, in which a belief is assumed to be true just because it originated some time ago. “We do not get enough sleep nowadays. Just a few centuries ago, people used to sleep for nine hours a night.”

• Equivocation: exploits the ambiguity of language by changing the meaning of a word during the course of an argument and using the different meanings to support an ill-founded conclusion.

• How can you be against faith when you take leaps of faith all the time: making investments, trusting friends, and even getting engaged?

• False Dilemma: an argument that presents a limited set of two possible categories and assumes that everything in the scope of the dicussion must be an element of that set.

• In the war on fanaticism, there are no sidelines; you are either with us or with the fanatics

• Not A Cause for A Cause: assumes a cause for an event where there is no evidence that one exists.

• The recent earthquake was because we disobeyed the king

• Appeal to Fear: plays on the fears of an audience by imagining a scary future that would be of their making if some proposition were accepted without solid evidence

I ask all employees to vote for my chosen candidate in the upcoming election. If the other candidate wins, he will raise taxes and many of you will lose your jobs.

• Hasty Generalization: forms a conclusion from a sample that is either too small or too special to be representative

• Appeal to Ignorance: assumes a proposition to be ture simply because there is no evidence proving that it is false

It is impossible to imagine that we actually landed a man on the moon, therefore it never happened

• No True Scotsman: comes up after someone has made a general claim about a group of things and then been presented with evidence challenging that claim. Rather than revising their position or contesting the evidence, they dodge the challenge by arbitrarily redefining the criteria for membership in that group.

“Programmers have no social skills.” “But John is programmer, and he is not socially awkward at all.” “Yes, but John isn’t a true programmer.”

• Genetic Fallacy: an argument is either devalued or defended solely because of its origins

Of course he supports the union workers on strike; he is from the same village

• Guilt by Association: discredit an argument for proposing an idea that is shared by some socially demonized individual or group. My opponent is calling for a healthcare system that would resemble that of socialist countries. Clearly that would be unacceptable.

• Affirming the Consequent: takes this form: if A then C; hence A. The error lies in assuming that because the consequent is true, the antecedent must also be true People who go to college are successful. John is successful, hence he must have gone to college.

• Appeal to Hypocrisy: involves countering someone’s argument by pointing out that it conflicts with his or her own past actions or statements A panelist objected to a protest in London against corporate greed because of the protesters’ apparent hypocrisy, pointing out that while they professed to be against capitalism, they continued to use smartphones and buy coffee.

• Slippery Slope: attempts to discredit a proposition by arguing that its acceptance will undoubtedly lead to a sequence of events, one or more of which are undesirable. We shouldn’t allow people uncontrolled access to the internet. The next thing you know they will be frequenting pronographic websites, and soon enough, our entir moral fabric will disintegrate and we will be reduced to animals

• Appeal to the Bandwagon: uses the fact that many people believe in something as evidence that it must be true.

Most people in Galileo’s day believed that the sun and the planets orbited around Earth, so Galileo faced ridicule for his support of the Copernican model.

• Ad-hominem ([æd’hɔminem]针对个人): attacks a person rather than the argument he or she is making, with the intention of diverting the discussion and discrediting their argument. You’re not a historian; why don’t you stick to your own field?

• Circular Reasoning: (aka begging the question ) assumes the conclusion in one or more of the premisses. A conclusion is either blatantly used as a premiss, or more often, it is reworded to appear as though it is a different proposition. You’re utterly wrong because you’re not making any sense.

• Composition and Division: becasue the parts of a whole have a particular attribute, the whole must have that attribute also.

Each module in this software system has been subjected to a set of unit tests and passed them all. Therefore, when the modules are integrated, the software system will not violate any of the invariants verified by those unit tests.

• Some Definitions:
• Proposition: A statement that is either true or false, but not both
• Argument: A set of propositions aimed at persuading through reasoning. In an argument, a subset of propositions, called premises , provides support for some other propositions called conclusions.
• Formal fallacy: An error in reasoning that is illogical because its structure is faulty. The fallacy can be spotted just by analyzing the argument’s form, without needing to evaluate its content.
• Informal fallacy: An error in reasoning that is illogical due to its content and context rather than its form. The error ought to be a commonly invoked one to be considered an informal fallacy.
• Deductive Argument: An argument in which if the premises are true, then the conclusion must be true. The conclusion is said to follow with logical necessity from the premises.

All men are mortal. Socrates is a man. Therefore, Socrates is mortal.

- valid: A deductive argument is valid if its conclusion does in fact follow logically from its premises. Otherwise, it is said to be in valid. The descriptors “valid” and “invalid” apply only to arguments and not to propositions.
- sound: A deductive argument is sound if it is valid and its premises are true. If either of those conditions does not hold, then the argument is unsound. Truth is determined by looking at whether the argument’s premises and conclusions are in accordance with facts in the real world.
- Inductive Argument: An argument in which if the premises are true, then it is probable that the conclusion will also be true. The conclusion does not follow from the premises with logical necessity, but rather with probability. Inductive arguments usually proceed from specific instances to the general.

Every time we measure the speed of light in a vacuum, it is 3×10^8m/s. Therefore, the speed of light in a vacuum is a universal constant.

- Strong: An inductive argument is strong, if its premises are true then it is highly probable that its conclusion is also true. Otherwise, if it is improbable that its conclusion it true, then it is said to be weak. Because they rely on probability, inductive arguments are not truth-preserving; it is never the case that a true conclusion must follow from true premises.
- cogent: An inductive argument is cogent if it is strong and the premises are actually true—that is, in accordance with facts. Otherwise, it is said to be uncogent.