Reductio ad absurdum

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Reductio ad absurdum is a type of logical argument where one assumes a claim for the sake of argument, arrives at an "absurd" result (often a contradiction), and then concludes that the original assumption must have been wrong, since it led to this absurd result.

Note that this is a logically valid technique. It is a form of modus tolens, an inference rule which takes this form:

  • If P then Q.
  • Q is false.
  • Therefore P is false.

More formally, a reductio ad absurdum argument typically takes the form:

  • Assume P.
  • This implies Q.
  • It also implies R.
  • But Q and R are contradictory (Q iff not R).
  • Therefore P is false.

Here's a real example (from number theory) of this type of argument in action:

  • Assume there are finitely many prime numbers.
  • Then there is a largest prime number. Call it p.
  • Consider the product of all the prime numbers up to this largest prime: 2 × 3 × 5 × 7 × 11 × ... × p.
  • Add one: (2 × 3 × 5 × 7 × 11 × ... × p) + 1. Call this number N.
  • Notice that N is not divisible by any of the primes up to p, since the remainder, when N is divided by any the primes, will always be 1. (For example, N divided by 2 gives the quotient 3 × 5 × 7 × 11 × ... × p, with remainder 1.)
  • So N is prime. And clearly, N is larger than p.
  • Thus we have found a prime number larger than the supposed "largest prime". This is a contradiction.
  • Therefore, we have to reject our original assumption. There must be infinitely many prime numbers.


See Can God create a rock so heavy that he can't lift it? for an example in the context of counter-apologetics (the claim being assumed is that God is all-powerful).

The problem with this type of argument is that the "absurdity" one reaches must actually be a logical contradiction in order for the argument to be valid. If the conclusion is simply unlikely, then the argument doesn't necessarily work. For example:

  • If God doesn't exist, then life arose by purely natural means.
  • This is absurd (read: very, very unlikely).
  • Therefore, God exists.

Well... no. Ignoring the fact that the premise is faulty (as its possible, however unlikely that life arose by something other than purely natural means) the absurdity is not shown. The assertion that its absurd is only used to come to the conclusion that God did it. The person that would be making this argument does not know how life arose, so to assume anything other than a deity seems absurd. (See: God of the gaps)

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