Difficulty understanding a logical equivalence

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Discussion Overview

The discussion centers around the logical equivalence between the expressions \(\exists{x}(P(x)\Rightarrow{Q(x)})\) and \(\forall{x}P(x)\Rightarrow{\exists{x}Q(x)}\). Participants explore the semantic verification of this equivalence through examples and logical reasoning.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant presents an example involving days, temperature, and snowfall to illustrate their difficulty in understanding the equivalence.
  • Another participant argues that if it is cold every day and there is at least one snowy day, then that snowy day must also be cold, suggesting that there exists a day where cold implies snow.
  • A third participant questions how the conclusion \(\exists{x}(P(x)\wedge{Q(x)})\) relates to the original expression \(\exists{x}(P(x)\Rightarrow{Q(x)})\).
  • A fourth participant states that if both \(P(x)\) and \(Q(x)\) are true for a particular \(x\), then it follows that \(P(x) \Rightarrow Q(x)\) is also true.

Areas of Agreement / Disagreement

Participants express differing views on the logical implications of the statements, indicating that the discussion remains unresolved with multiple competing interpretations of the equivalence.

Contextual Notes

Participants have not fully clarified the assumptions underlying their interpretations, and there are unresolved steps in the logical reasoning presented.

Who May Find This Useful

This discussion may be of interest to those studying symbolic logic, particularly in understanding logical equivalences and their semantic interpretations.

zelmac
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\exists{x}(P(x)\Rightarrow{Q(x)})\equiv{\forall{x}P(x)\Rightarrow{\exists{x}Q(x)}}

I am able to derive this equivalence by using the standard equivalences of symbolic logic, but when I try to verify this semantically, with an example, I just can't see why these two expressions are equivalent.

Example:
Lets say that x represents days, P(x) represents it's cold on day x, and Q(x) represents it is snowing on day x. If it is true that it is cold every day, and it is true that there is a snowy day, why must it be true that there is a day when, if it is cold, than snow MUST fall?
 
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If it is cold every day, and there is at least one snowy day, this snowy day has to be cold. Therefore, there is a day (∃) where it is cold and snow falls.
 
You concluded:
\exists{x}(P(x)\wedge{Q(x)})
and I'm wondering how do you get to:
\exists{x}(P(x)\Rightarrow{Q(x)})
 
If P(x) ∧ Q(x) is true for this x, then P(x) => Q(x).
 

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