Difficulty understanding a logical equivalence

AI Thread Summary
The discussion centers on the logical equivalence between the expressions ∃x(P(x) ⇒ Q(x)) and ∀xP(x) ⇒ ∃xQ(x). The user struggles to understand this equivalence semantically, particularly through an example involving days, temperature, and snowfall. They argue that if it is cold every day and there is at least one snowy day, it seems to imply that there is a specific day where coldness guarantees snowfall. The confusion arises in transitioning from the conjunction of cold and snow to the implication form, leading to questions about the validity of the logical steps. Ultimately, the thread highlights the complexities of interpreting logical equivalences in practical scenarios.
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).
 
I was reading documentation about the soundness and completeness of logic formal systems. Consider the following $$\vdash_S \phi$$ where ##S## is the proof-system making part the formal system and ##\phi## is a wff (well formed formula) of the formal language. Note the blank on left of the turnstile symbol ##\vdash_S##, as far as I can tell it actually represents the empty set. So what does it mean ? I guess it actually means ##\phi## is a theorem of the formal system, i.e. there is a...
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