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.
Examples:
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.