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