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The liar paradox explained, with Russell's and the sorites paradox

The liar paradox combines self-reference with negation, creating a loop that challenges the principle of bivalence. The cards cover Russell's paradox, the sorites paradox, modal logic and possible worlds, the principle of explosion, and affirming the consequent. Try the cards to see how these concepts connect.

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A necessary truth is one that remains true no matter how much the world around us changes.

From the Knowverse course text.

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The box symbol looks like a small square (☐) and is placed before a statement to show it is necessary.

From the Knowverse course text.

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A Possible World is not just a single changed event; it is a total, finished version of history.

From the Knowverse course text.

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When we combine self-reference with negation (the 'false' part), we create a loop that logic cannot settle.

From the Knowverse course text.

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The rule for this Monster Catalog is simple: it will list every catalog in the library that does not list itself.

From the Knowverse course text.

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This puzzle is known as the Sorites Paradox, named after the Greek word for 'heap'.

From the Knowverse course text.

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This is a fallacy because having the result (Q) doesn't prove that the specific cause (P) was the one that triggered it.

From the Knowverse course text.

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This 'explosion' means that if you accept one contradiction, you can logically prove that *anything* is true, making the system useless.

From the Knowverse course text.

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Modal logic and possible worlds

Standard logic evaluates whether statements are true based on current facts. Modal logic introduces deeper layers of meaning regarding how stable those truths are through necessity and possibility.

Logicians use the concept of possible worlds as complete, finished versions of history to evaluate whether statements hold true across alternative scenarios.

Things that are necessary remain true no matter how much the world changes around us. By using specific symbols, modal logic allows thinkers to distinguish between accidental facts and fundamental realities.

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Logicians call these different 'ways things could have been' Possible Worlds.

From the Knowverse course text.

Bertrand Russell — image from Wikimedia Commons
Bertrand Russell transparent bg · Honourable Bertrand Russell.jpg: Photographer not identified derivative work: Conquistador · Public domain

Paradoxes and system limits

Self-reference combined with negation creates loops that break classical logic. The liar paradox and russell's paradox demonstrate how loose definitions can cause logical systems to collapse.

Thinkers developed specialized frameworks like type theories and many-valued logics to prevent contradictions from invalidating mathematical and computational foundations.

When a sentence talks about its own truth value while incorporating negation, it creates a cycle that challenges the basic principle that statements must be strictly true or false.

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The paradox arises because the sentence uses self-reference, meaning it refers back to its own truth value.

From the Knowverse course text.

Fallacies and system rules

Valid arguments require proper structural form so that true starting points guarantee true conclusions. Formal fallacies occur when the underlying arrangement of statements contains an error in reasoning.

Common mistakes like affirming the consequent happen when someone assumes that finding a result proves a specific cause occurred. Logical analysis helps separate valid proofs from likely guesses.

Modern computer systems build upon these formal rules using logic gates and automated theorem proving. Understanding these principles helps engineers design reliable software and database structures.

Source excerpt
This is a fallacy because having the result (Q) doesn't prove that the specific cause (P) was the one that triggered it.

From the Knowverse course text.

Sources

Sources: Advanced Topics and Logical Paradoxes. Each question or step includes an excerpt from the course.