What Does the Logic Axiom of Simplification Mean?

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Discussion Overview

The discussion revolves around the logic axiom of simplification, specifically the expression {a} → ({b} → {a}). Participants seek to clarify its meaning and implications, exploring both its intuitive understanding and formal equivalences.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses difficulty in finding a simple explanation for the axiom, suggesting it is often considered obvious.
  • Another participant argues that if a is false, the statement becomes "false implies stuff," which is true by definition, and if a is true, it reduces to "b implies true," which is also true by definition.
  • A participant presents a formal equivalence of the axiom, stating that it is equivalent to (a & b) → a, and provides a series of logical transformations to support this claim.
  • Another participant reiterates the equivalence and provides additional reasoning based on assumptions about implications and the structure of logical formulas.
  • One participant suggests that the axiom expresses the idea that a is deducible from a and b, framing it as an instance of the structural rule of weakening.

Areas of Agreement / Disagreement

Participants present multiple interpretations and explanations of the axiom, with no clear consensus on a singular understanding. Some agree on the formal equivalences, while others focus on intuitive meanings.

Contextual Notes

Some participants reference logical transformations and structural rules, but the discussion does not resolve the nuances of these interpretations or their implications.

matheinste
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Hello all

I cannot find a simple explanation of the meaning of this axiom, probably because it is considered so obvioius that it needs no explanation. Can anyone explain in words.

[tex]{a}\rightarrow{({b}\rightarrow{a})}[/tex]

Thanks. Matheinste.
 
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I'm not sure what you're looking for.

If a is false, then the statement reduces to "false implies stuff" which is by definition true. If a is true the statement reduces to "b implies true" which is also by definition true.
 
a -> (b -> a) is also equivalent to (a & b) -> a:

a -> (b -> a)
~a v (~b v a) [p -> q <=> ~p v q]
(~a v ~b) v a [(p v q) v r <=> p v (q v r)]
~(a & b) v a [~(p & q) <=> ~p v ~q]
(a & b) -> a

This formula also follows from the assumptions that (i) a formula always implies itself (p -> p) and (ii) lengthening a formula doesn't remove any of the formulas that the original implied ((p -> q) -> ((p & r) -> q)).
 
honestrosewater said:
a -> (b -> a) is also equivalent to (a & b) -> a:

a -> (b -> a)
~a v (~b v a) [p -> q <=> ~p v q]
(~a v ~b) v a [(p v q) v r <=> p v (q v r)]
~(a & b) v a [~(p & q) <=> ~p v ~q]
(a & b) -> a

This formula also follows from the assumptions that (i) a formula always implies itself (p -> p) and (ii) lengthening a formula doesn't remove any of the formulas that the original implied ((p -> q) -> ((p & r) -> q)).

Thanks also to GRGreathouse. I see it now.

To Compuchip. Yes, I mistakenly repeated the thread but did not know how to remove the second posting.

Thanks. Matheinste.
 
matheinste said:
Hello all

I cannot find a simple explanation of the meaning of this axiom, probably because it is considered so obvioius that it needs no explanation. Can anyone explain in words.

[tex]{a}\rightarrow{({b}\rightarrow{a})}[/tex]

Thanks. Matheinste.
It's a formula expressing the fact that a is deducible from a,b. It's a particular instance of the structural rule of weakening (which says that if A |- B, then A,phi |- B).
 

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