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These sentences sound illogical. Their arguments can fall to pieces easily.

  • "God exists because The Bible says so, and The Bible is the word of God."
  • "Whatever is less dense than water will float, because such objects won't sink in water.
  • "If you don't believe me, ask Mary. I vouch for her integrity."

Is there a word to describe such reasoning?

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    Yes: "circular reasoning", or "tautology". When it's less blatant, you could call it "begging the question", "assuming the consequent", or "petitio principii", which are all synonyms.
    – Dan Bron
    Commented Sep 11, 2014 at 18:30
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    Circular - except the dog fighting reference. That's just nonsense.
    – Kristina Lopez
    Commented Sep 11, 2014 at 18:33
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    @DanBron You should add that as an answer. Begging the question and circular reasoning are probably the best two answers there is for this.
    – Janus Bahs Jacquet
    Commented Sep 11, 2014 at 18:45
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    All logical fallacies are types of non sequiturs. These examples can be covered by a number of fallacies. See any good list of logical fallacies and pick your own.
    – anongoodnurse
    Commented Sep 11, 2014 at 19:44
  • I actually think the original second sentence was much better than the one you've substituted with. By the way, isn't the "sink in water" statement true? Anything which is less dense than water will not sink. Help?!
    – Mari-Lou A
    Commented Sep 12, 2014 at 4:57

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This type of argumentation is called "circular reasoning" or "tautological".

When the circularity of the argument is less blatant, and the tautology is indirect or merely implied by one of the premises, then the fallacy is known as "begging the question".

In recent years, "begging the question" has started to be used as a synonym for "raising the question", so if you wanted to avoid ambiguity, or use the term in a more formal context (e.g. philosophy, rhetoric), you could use one of the synonyms "assuming the consequent" or "petitio principii".

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Circular reasoning is really the correct term. Technically, this is also "begging the question," but as was previously pointed out, common (mis)use of that term has introduced ambiguity. Although the term "tautology" is often used to describe this type of situation, tautologies and circular reasoning are not necessarily the same thing. A tautology is a statement which is true by definition, including those which do not involve circular reasoning. A tautology may not be a logical flaw. For instance, it is common practice for mathematical proofs to begin include some tautologies among the postulates that are used to prove the theory.

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