T5.4 4 of 4

We may restate our strategy guideline. In the last exercise it was suggested that one assume the "opposite" of the goal sentence. Let me make the suggestion more explicit.

When you have a goal sentence of the form '~P', think about deriving it from negation introduction. This would require assuming P and then (sub)deriving a contradiction for ~I.

When you have a goal sentence P, that is, a sentence of any form, and you see no other way to derive it, think about assuming '~P' deducing a contradiction and applying ~E.

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