11148
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Deduction is when we suppose one thing, and another necessarily follows
[Aristotle]
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Full Idea:
A deduction is a discourse in which, certain things having been supposed, something different from the things supposed results of necessity because these things are so.
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From:
Aristotle (Prior Analytics [c.328 BCE], 24b18)
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A reaction:
Notice that it is modal ('suppose', rather than 'know'), that necessity is involved, which is presumably metaphysical necessity, and that there are assumptions about what would be true, and not just what follows from what.
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22279
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Frege's sign |--- meant judgements, but the modern |- turnstile means inference, with intecedents
[Potter]
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Full Idea:
Natural deduction systems generally depend on conditional proof, but for Frege everything is asserted unconditionally. The modern turnstile |- is allowed to have antecedents, and hence to represent inference rather than Frege's judgement sign |---.
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From:
Michael Potter (The Rise of Analytic Philosophy 1879-1930 [2020], 03 'Axioms')
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A reaction:
[compressed] Shockingly, Frege's approach seems more psychological than the modern approach. I would say that the whole point of logic is that it has to be conditional, because the truth of the antecedents is irrelevant.
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10752
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Γ |- S says S can be deduced from Γ; Γ |= S says a good model for Γ makes S true
[Rossberg]
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Full Idea:
Deductive consequence, written Γ|-S, is loosely read as 'the sentence S can be deduced from the sentences Γ', and semantic consequence Γ|=S says 'all models that make Γ true make S true as well'.
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From:
Marcus Rossberg (First-order Logic, 2nd-order, Completeness [2004], §2)
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A reaction:
We might read |= as 'true in the same model as'. What is the relation, though, between the LHS and the RHS? They seem to be mutually related to some model, but not directly to one another.
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