Friday, August 19, 2016

Today's Logical Fallacy is...Tautology!

A tautology is a statement that – by it’s construction - must always be true. It uses circular reasoning in that it’s conclusion is its own premise. While this type of logic can be easy to spot (“the Bible is the Word of God because it says so in the Bible”), it can be deceptive, especially when you’re presented with terms with which you are unfamiliar (“therapeutic touch works because it manipulates life force” – the definition of “therapeutic touch” is the alleged manipulation of life force, so it’s like saying that breathing keeps you alive because it exchanges carbon dioxide for oxygen). (Note: Definitions and mathematical proofs are not “arguments,” so while they meet the qualifications to be called tautologies, they aren’t tautological fallacies.) Tautologies appear to be explanations but actually provide no useful information. They are also unfalsifiable since they are entirely dependent on their own premise.


You are a disagreeable person and, if you disagree with me on this, it will only further prove what a disagreeable person you are.

The ideas of Ayn Rand are considered by many philosophers to be tautological:
David Bentley Hart (“First Things”):
"Rand was so eerily ignorant of all the interesting problems of ontology, epistemology, or logic that she believed she could construct an irrefutable system around a collection of simple maxims like "existence is identity" and "consciousness is identification," all gathered from the damp fenlands between vacuous tautology and catastrophic philosophical error. She was simply unaware that there were any genuine philosophical problems that could not be summarily solved by flatly proclaiming that this is objectivity, this is rational, this is scientific, in the peremptory tones of an Obersturmfuhrer drilling his commandoes."

Either it will rain tomorrow or it won’t rain.


  1. Scientist perhaps, but not grammarian. Please refresh your understanding of when to use an apostrophe in the word its or it's. You said: It's construction (wrong), it's conclusion (again wrong!), its premise (correct - you finally got it right!). They're all possessives, so why did you punctuate them differently? Your article and the message in it would be so much more convincing if your own writing followed the rules of grammar and punctuation. Just a thought....

    1. this comment is the definition of pedantic

    2. Yeah, we don't want pedants schooling people on grade school points of punctuation.

  2. This was my understanding of what a tautology was too. I even have a textbook on the principles of reasoning written by an esteemed professor of philosophy, Thinking Straight by Monroe C Beardsley that makes similar comments to yours. But today I was told by a student of propositional logic that strictly speaking true by definition arguments are not considered tautologies. That this is just a kind colloquial idea of tautology, but not technically correct.

    He provided me with the following definition:
    "The usual terminology is that a tautology is an assertion of propositional logic that is true in every row of the truth table, that, is every propositional world."

    Unfortunately I am not trained in formal logic so I wasn't really able to understand how true by definition arguments fail to meet this criteria.

    Is this student correct; what do you think?

  3. This is an incorrect definition of tautology as I understand it in the context of logic. A tautology itself isn’t fallacious, and isn’t the same as circular logic. X=y y=x or the ball is green or it is not green are examples of tautologies. These are statements true by virtue of their logical form, whereas circular reasoning or logic is in the form of an argument where you’re assuming the conclusion based on a premise.