A logical fallacy is an error of reasoning. It either has an error in the logical structure of deduction (formal fallacy), or is falsely inducted from one or more of its premises (informal fallacy).
I will list a few common fallacies here.
In the fallacy of circular reasoning, you assume to be true what you are supposed to be proving.
This product is the best because it is better than any other products.
A more complicated example:
A: This product is perfect.
B: But there is an article in yesterdayâ€™s newspaper saying that there is a defect in this product.
A: That article must be wrong. How could a perfect product have a defect?
Post hoc reasoning
Also called false cause reasoning. It argues because A preceded B, therefore A caused B.
All plants that survive in metal soils produce histidine. Histidine must be the reason that they can survive the harmful soil.
My red shirt is lucky. Every time I wear it in a soccer game my team wins.
A related fallacy is called Gamblers fallacy
An argument that assumes that a departure from what occurs on average or in the long term will be corrected in the short term.
In a coin toss experiment, the result for the first five throw is head. Therefore we have a very high probability of getting a tail in the six throw.
An argument that falsely assume that one thing must lead to another.
Company A has been growing much faster than company B in the past five years. Soon A will surpass B.
You can never give anyone a break. If you do, they'll walk all over you.
A question constructed in such a way that agreement or disagreement with one term seems to imply agreement with the second.
Are you going to admit that youâ€™re wrong?
Have you enjoyed spoiling the dinner for everybody?
Informal Relevance Fallacy
Argument by authority
An argument that bases the judgment of an assertion on the source of the assertion.
This medicine is effective because my doctor said so.
A related fallacy is sometimes called Personal attack
, in which one dispute an argument based on the source of the argument.
Jerry recommended this milk. But I distrust Jerry. So I wonâ€™t buy this milk.
Appeal to common belief
A fallacious argument that concludes a proposition to be true because many or all people believe it.
Since 60% of the people polled believed in the Bible, the Bible must be true.
Fifty million Elvis fans can't be wrong.
Doctors recommended this medicine than any other medicine. You should buy it.
Brand X vacuums are the leading brand in America. You should buy Brand X vacuums.
A similar type of fallacy is Appeal to tradition
A common logical fallacy in which someone proclaims his or her accuracy by noting that "this is how it's always been done."
This is right because we've always done it this way.
Your invention is a bad idea because it has no historical precedent.
Argument from ignorance
A logical fallacy in which it is claimed that a premise is true only because it has not been proven false, or that a premise is false only because it has not been proven true.
Nobody has seen it. It could not have happened.
Itâ€™s similar to Shifting the Burden of Proof
The God exist. Unless you can prove that the God doesnâ€™t exist, you have to accept it as the truth.
Itâ€™s also similar to Argument from incredulity
What he said is very hard to believe. It simply cannot be true.
An argument that bases a generalization on insufficient number of instances.
The weather must be perfect in San Diego all the time. Iâ€™ve been there several times, and the sky was always blue and the temperature ideal.
Three of my patients have been harmed by this vaccine. It must be unsafe.
A reverse type of fallacy is Sweeping generalization
, which applies a general rule to a specific case.
Our GMAT class has raised the participants an average of 100 points in their GMAT scores. You had a GMAT score of 600 and have taken our class. You will score 700 in your next attempt for sure.
The conclusion is drawn based on a biased sample that is not representative of the entire population.
I canâ€™t believe our restaurant isnâ€™t the best in town. All our regular customers love us.
the fallacy of the alternative disjunct
A or B
Therefore not B
Number x belongs to a set that contains numbers that are multiples of 2 or 3. It is a multiple of 2, therefore it must not be a multiple of 3.
Affirming the consequent
If P, then Q.
All humans are mortal. Meow is mortal. She must be a human.
(She could be a cat.)
Argument from fallacy
If A then B
Therefore not B
I close my eyes when Iâ€™m sleeping. Iâ€™m not sleeping, so my eyes must not be closed.
When I was little I was told that an Angel always pushes a dropped pencil down to the floor. Now I know that angels do not exist. Therefore a pencil will never drop down to the floor.
Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.