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I am new to natural deduction and upon reading about various methods online, I came across the rule of bottom-elimination in the following example.

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I do not understand the step in line 10.

Upon inspection, my initial thought would be that the assumption of ¬p and p both being true is absurd, hence anything can be inferred ( in this case 'p'). However, if this were the case where would you stop (this seems to be an overly powerful tool)? So I assume that this idea is wrong.

Could someone help me understand the rule?

NOTE: I came to StackExchange due to the lack of resources and specific information online.

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  • $\begingroup$ This strikes me as more appropriate for Mathematics. Perhaps it should be migrated there? $\endgroup$
    – Yuval Filmus
    May 10, 2020 at 6:34

1 Answer 1

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Usually in practice we weld the two steps together and just say that from $p$ and $\lnot p$ anything follows, but in formal logic this is a combination of two rules of inference:

  1. $p$ and $\lnot p$ both together entail falsehood $\bot$,
  2. from $\bot$ anything follows.

These are precisely lines 9 and 10 in your proof.

We often take $\lnot p$ to be an abbreviation for $p \Rightarrow \bot$, in which case the rule "from $p$ and $\lnot p$ follows $\bot$" is just a special case of modus ponens "from $p$ and $p \Rightarrow r$ follows $r$".

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  • $\begingroup$ Please excuse my lack of understanding, but for clarification, does step 2. of your answer imply that ANYTHING at all can be made true? If so, can't things that aren't true be made true just by assuming p and ¬p i.e. can't everything be made true by the repeated use of p & ¬p? $\endgroup$
    – Newbie123
    May 9, 2020 at 16:47
  • $\begingroup$ Anything can be made true if you already proved or assumed $\bot$. And you cannot "repeatedly use $p$ and $\lnot p$" unless you already know that they are both true. $\endgroup$
    – Andrej Bauer
    May 9, 2020 at 16:49
  • $\begingroup$ In fact that is the whole point of rule 2: if you know something that is false (namely $\bot$) then you can conclude anything you like. The rule does not say "everything is true"! $\endgroup$
    – Andrej Bauer
    May 9, 2020 at 16:51

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