"Extraordinary claims require extraordinary evidence." -- Carl Sagan

"What can be asserted without evidence can also be dismissed without evidence." -- Christopher Hitchens

What is a logical fallacy? All arguments have the same basic structure: A therefore B. They begin with one or more premises (A), which is a fact or assumption upon which the argument is based. They then apply a logical principle (therefore) to arrive at a conclusion (B). An example of a logical principle is that of equivalence. For example, if you begin with the premises that A=B and B=C, you can apply the logical principle of equivalence to conclude that A=C. A logical fallacy is a false or incorrect logical principle. An argument that is based upon a logical fallacy is therefore not valid. It is important to note that if the logic of an argument is valid then the conclusion must also be valid, which means that if the premises are all true then the conclusion must also be true. Valid logic applied to one or more false premises, however, leads to an invalid argument. Also, if an argument is not valid the conclusion may, by chance, still be true. For a more thorough discussion of logical fallacies and how to structure a logical argument, see the New England Skeptical Society's article, How To Argue.

Top 20 Logical Fallacies (in alphabetical order) taken from -

The Skeptics' Guide To The Universe

The New England Skeptical Society The Skeptics' Guide To The Universe Science-Based Medicine
The James Randi Educational Foundation
Propaganda
snopes.com
Bad Science by Ben Goldacre
Simon Singh and his fight to mend British libel laws
Professor Richard Wiseman
PZ Myers' Pharyngula
Dr. Phil Plait and Bad Astronomy
Skeptic Blog
Michael Shermer's Baloney Detection Kit
The Demon-Haunted World: Science as a Candle in the Dark by Carl Sagan
The Skeptics' Dictionary
List of cognitive biases
Don Watson - Weasel Words
QuackWatch
A List of Fallacious Arguments

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"No skeptic, to my knowledge, ever made a major scientific discovery or advanced the welfare of others."

-- Deepak Chopra (yes, he actually said that)

"The most exciting phrase to hear in science, the one that heralds the most discoveries, is not "Eureka!" (I found it!) but That's funny..." -Isaac Asimov



The scientific world view is full of awe and wonder. Understanding how truly awesome the universe is – in its elegant complexity, its staggering beauty, and the many intricate systems of which it is comprised – gives a profound feeling of connectedness and sparks the imagination. And it has the advantage of being real. -Steven Novella



The glory which is built upon a lie soon becomes a most unpleasant incumbrance. How easy it is to make people believe a lie, and how hard it is to undo that work again.
— Mark Twain