Slide 1
A conclusion needs support
An argument connects premises, the starting claims, to a conclusion. Logic tests whether the premises support the conclusion.
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Imagine a museum rule: every red ticket admits its holder, and your ticket is red. The conclusion follows even before we investigate whether the museum really has that rule.
Slide 2
Try to break the connection
A deductively valid argument cannot have true premises and a false conclusion. Validity describes this relationship between claims.
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To challenge the museum inference, try to keep both starting claims true while making admission false. You cannot. Changing the rule challenges a premise instead, which is a different objection.
Slide 3
A familiar mistake
If a red ticket admits you, admission does not prove that your ticket was red. Other tickets might also admit their holders.
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Consider a blue ticket accepted at the same entrance. It is a counterexample—an example that disproves the reversed inference—without contradicting the original red-ticket rule. A plausible conclusion can still lack support.
Slide 4
Validity is not enough
A sound argument combines valid reasoning with true premises. Perfectly arranged reasoning cannot rescue a false starting claim.
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A printed rule might be outdated. Evidence gathering checks whether the rule is still in force. Logic checks whether the conclusion follows from it. Good reasoning requires both.
Slide 5
Disagree at the right place
When assessing an argument, distinguish a disputed premise from a failed inference. They need different kinds of repair.
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A critic may accept the conclusion but reject the offered reason for it. Replacing that reason can strengthen a shared position; exposing a bad argument alone does not establish the opposite conclusion.
Sources
- Classical Logic
Stanford Encyclopedia of Philosophy
- Validity and Soundness
Internet Encyclopedia of Philosophy