19 lines
682 B
Markdown
19 lines
682 B
Markdown
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---
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tags:
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- propositional-logic
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- logic
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---
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# Disjunction Introduction
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This rule can seem a little odd: like we are randomly introducing an additional
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proposition without giving any justification. However this is just a consequence
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of the fact if $P$ is true, so is $P \lor Q$ since disjunction is not the same
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as conjunction: only one disjunct needs to be true for the compound disjunction
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to be true. This is represented in the context of
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[truth-trees](Truth-trees.md#disjunction-decomposition) by the fact that truth
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can pass up via either branch of a disjunction pattern. This rule is sometimes
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also referred to (confusingly) as _Addition_.
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