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E.g. Modus Ponens (MP)
φ → ψ φ ψ
P →(Q →R) P Q →R
raining → wet raining wet
(P →Q) →R P → Q R
wet → slippery wet slippery
I.e. we can substitute for the
metavariables complex sentences
Note that, by stringing together
applications of rules of inference, it is
possible to derive conclusions that cannot
be derived in a single step. This idea of
stringing together rule applications leads
to the notion of a proof.