First:
What logic isn't
It's worth mentioning a couple of things which logic is not.
Firstly, logical reasoning is not an absolute law which governs the universe. Many times in the past, people have concluded that because something is logically impossible (given the science of the day), it must be impossible, period. It was also believed at one time that Euclidean geometry was a universal law; it is, after all, logically consistent. Again, we now know that the rules of Euclidean geometry are not universal.
Secondly, logic is not a set of rules which govern human behavior. Humans may have logically conflicting goals. For example:
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John wishes to speak to whoever is in charge.
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The person in charge is Steve.
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Therefore John wishes to speak to Steve.
Unfortunately, John may have a conflicting goal of avoiding Steve, meaning that the reasoned answer may be inapplicable to real life.
This document only explains how to use logic; you must decide whether logic is the right tool for the job. There are other ways to communicate, discuss and debate.
Next, Thirty-eight dishonest tricks which are commonly used in argument, with the methods of overcoming them so you know when you're getting played. This crap happens in newspapers and such, not just in blog comments.
Emphasis added to my personal favorites. Not like you have to memorize stuff...
This is taken from "Straight and crooked thinking" by Robert H. Thouless, Pan Books, ISBN 0 330 24127 3, copyright 1930, 1953 and 1974. Heartily recommended.
- The use of emotionally toned words (pp 10-25)
Dealt with by translating the statement into words emotionally neutral
- Making a statement in which "all" is implied but "some" is true (pp 27-38)
Dealt with by putting the word "all" into the statement and showing that it is then false.
- Proof by selected instances (pp 32-37)
Dealt with dishonestly by selecting instances opposing your opponent's contention or honestly by pointing out the true form of the proof (as a statistical problem in association) and either supplying the required numerical facts or pointing out that your opponent has not got them.
- Extension of an opponent's proposition by contradiction or by misrepresentation of it (pp 39-43)
Dealt with by stating again the more moderate position which is being defended.
- Evasion of a sound refutation of an argument by the use of a sophistical formula (pp 41-44)
Dealt with by analysis of the formula and demonstration of its unsoundness.
- Diversion to another question, to a side issue, or by irrelevant objection (pp 44-48)
Dealt with by refusing to be diverted from the original question, but stating again the real question at issue.
- Proof by inconsequent argument (pp 49-50)
Dealt with by asking that the connection between the proposition and the alleged proof may be explained, even though the request for explanation may be attributed to ignorance or lack of logical insight on the part of the person making it.
- The argument that we should not make efforts against X which is admittedly evil because there is a worse evil Y against which our efforts should be directed (pp 50-52)
Dealt with by pointing out that this is a reason for making efforts to abolish Y, but no reason for not also making efforts to get rid of X.
- The recommendation of a position because it is a mean between two extremes (pp 52-54)
Dealt with by denying the usefulness of the principle as a method of discovering the truth. In practice, this can most easily be done by showing that our own view also can be represented as a mean between two extremes.
- Pointing out the logical correctness of the form of an argument whose premisses contain doubtful or untrue statements of fact (p 58)
Dealt with by refusing to discuss the logic of the argument but pointing out the defects of its presentations of alleged fact.
- The use of an argument of logically unsound form (pp 58-64)
Since the unsoundness of such arguments can be easily seen when the form of the argument is clearly displayed, an opponent who does this can be dealt with by making such a simple statement of his argument that its unsoundness is apparent. For one's own satisfaction when reading an argument of doubtful soundness, it will often be found useful to make a diagram.
- Argument in a circle (p 64)
- Begging the question (pp 65-66)
Both 12 and 13 can be dealt with in the same way as 11; by restating your opponent's argument in such a simple way that the nature of the device used must be clear to anyone.
- Discussing a verbal proposition as if it were a factual one, or failing to disentangle the verbal and factual elements in a proposition that is partly both (pp 67-77)
This is really an incompetent rather than a dishonest way of arguing. The remedy is to point out how much of the question at issue is a difference in the use of words and how much (if at all) it is a difference as to fact or values. - Putting forward a tautology (such as that too much of the thing attacked is bad) as if it were a factual judgement (pp 71-72)
Dealt with by pointing out that the statement is necessarily true from its verbal form.
- The use of a speculative argument (pp 78-83)
Rebutted by pointing out that what is cannot be inferred from what ought to be or from what the speaker feels must be.
- Change in the meaning of a term during the course of an argument (pp 88-94)
Dealt with by getting the term defined or by substituting an equivalent form of words at one of the points where the term in question is used and seeing whether the use of this form of words will make true the other statements in which this term is used.
- The use of a dilemma which ignores a continuous series of possibilities between the two extremes presented (pp 103-105)
Dealt with by refusing to accept either alternative, but pointing to the fact of the continuity which the person using the argument has ignored. Since this is likely to appear over-subtle to an opponent using the argument, it may be strengthened by pointing out that the argument is the same as saying, "Is this paper black or white?" when it is, in fact, a shade of grey.
- The use of the fact of continuity between them to throw doubt on a real difference between two things (the "argument of the beard") (pp 105-108)
Dealt with by pointing out that the difference is nevertheless real. This again may be made stronger by pointing out that application of the same method of argument would deny the difference between "black" and "white" or between "hot" and "cold".
- Illegitimate use of or demand for definition (p 109)
If an opponent uses definitions to produce clear-cut conceptions for facts which are not clear-cut, it is necessary to point out to him how much more complicated facts are in reality than in his thought. If he tries to drive you to define for the same purpose, the remedy is to refuse formal definition but to adopt some other method for making your meaning clear.
- Suggestion by repeated affirmation (pp 111-114)
- Suggestion by use of a confident manner (pp 114-115)
- Suggestion by prestige (pp 115-118)
The best safeguard against all three of these tricks of suggestion is a theoretical knowledge of suggestion, so that their use may be detected. All three devices lose much of their effect if the audience see how the effect is being obtained, so merely pointing out the fact that the speaker is trying to create conviction by repeated assertion in a confident manner may be enough to make this device ineffective. Ridicule is often used to undermine the confident manner, or any kind of criticism which makes the speaker begin to grow angry or plaintive.
- Prestige by false credentials (pp 115-118)
The obvious remedy for this is, when practical, to expose the falsity of the titles, degrees, etc, that are used. The prestige then collapses.
- Prestige by the use of pseudo-technical jargon (pp 116-118)
Best dealt with by asking in a modest manner that the speaker should explain himself more simply.
- Affectation of failure to understand backed by prestige (pp 118-119)
Dealt with by more than ample explanation.
- The use of questions drawing out damaging admissions (pp 199-120)
Dealt with by refusal to make the admissions. The difficulty of this refusal must be overcome by any device reducing one's suggestibility to the questioner.
- The appeal to mere authority (pp 122-125)
Dealt with by considering whether the person supposed to have authority had a sound reason for making the assertion which is attributed to him.
- Overcoming resistance to a doubtful proposition by a preliminary statement of a few easily accepted ones (pp 128-130)
Knowledge of this trick and preparedness for it are the best safeguard against its effects.
- Statement of a doubtful proposition in such a way that it fits in with the thought- habits or the prejudices of the hearer (pp 133-135 and p 157)
A habit of questioning what appears obvious is the best safeguard against this trick. A particular device of value against it is to restate a questionable proposition in a new context in which one's thought-habits do not lead to its acceptance.
- The use of generally accepted formulae of predigested though as premisses in argument (pp 161-166)
The best way of dealing with predigested thinking in argument is to point out good- humouredly and with a backing of real evidence that matters are more complicated than your opponent supposes.
- "There is much to be said on both sides, so no decision can be made either way", or any other formula leading to the attitude of academic detachment (pp 166-167)
Dealt with by pointing out that taking no action has practical consequences no less real than those which result from acting on either of the propositions in dispute, and that this is no more likely than any other to be the right solution of the difficulty.
- Argument by mere analogy (pp 169-178)
Dealt with by examining the alleged analogy in detail and pointing out where it breaks down.
- Argument by forced analogy (pp 178-179)
The absurdity of a forced analogy can best be exposed by showing how many other analogies supporting different conclusions might have been used.
- Angering an opponent in order that he may argue badly (pp 146-147)
Dealt with by refusing to get angry however annoying our opponent may be.
- Special pleading (pp 154-156)
Dealt with by applying one's opponent's special arguments to other propositions which he is unwilling to admit.
- Commending or condemning a proposition because of its practical consequences to the bearer (pp 157-158)
We can only become immune to the effect of this kind of appeal if we have formed a habit of recognizing our own tendencies to be guided by our prejudices and by our own self-interest, and of distrusting our judgement on questions in which we are practically concerned.
- Argument by attributing prejudices or motives to one's opponent (p 159)
Best dealt with by pointing out that other prejudices may equally well determine the opposite view, and that, in any case, the question of why a person holds an opinion is an entirely different question from that of whether the opinion is or is not true.
