Formal Logic 1.2
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Propositional Wellformed Formulas (wffs)
Statements are sometimes called propositions.

Hypothesis
In an argument the statement or statements that preceed the conclusion.

Conclusion
In an argument what follows the hypothesis.

Valid Argument
 An argument that should be True based entirely on its internal structure or "intrinsicaly True."
 An argument whose conclusion should be related to or follow from the hypothesis.

Derivation Rules
 Rules that manipulate wellformed formulas (wffs) in a truthpreserving manner.
 Begin with the hypothesis (assumed True) and attempt to apply the manipulation rules in such a way as to end up with the conclusion (which must then also be True because truth is preserved under the rules).

Proof Sequence
A sequence of wellformed formulas (wffs) in which each wff is either a hypothesis or the result of applying one of the formal system's derivation rules to earlier wffs in the sequence.

Equivalence Rules
Rules that state that certain pairs of wellformed formulas (wffs) A and B are equivalent.

Inference Rules
Rules that say that if 1 or more wellformed formulas (wffs) that match the 1st part of the rule pattern are already part of the proof sequence, we can add to the proof sequence a new wff that matches the last part of the rule pattern.