We can rewrite formulas, with the aim of better understanding the meaning of the logical ‘bones’ (=form) of the statement, and to produce provable formulas, or proofs.
If the rewritten formula has the same meaning as its original formula, both formulas are equivalent (≡). If instead we place a biconditional (↔) between the rewritten and its original formula, and we calculate under this operand, then we get a tautology.
1. Same meaning
Rewriting formulas with operands ˅, ˄, ~
If we use the operands ˅, ˄, ~, which together form a functionally complete set, then we can rewrite these formulas using the following rules (for statements with 2 variables and 1 operand):
Rewriting the implication
Expressions that are often used to rewrite the implication.
2. Different meaning
Expressions opposing/contrary to the implication
expression |
form |
description of the operation |
outcome1) |
diagram |
inverse |
~p→~q |
negation of both statements |
|1101| |
|
converse |
q→p |
reversal of both statements |
|1101| |
Negating the implication
expression |
form |
description of the operation |
outcome1) |
diagram |
negation |
p˄~q |
contradicts the implication |
|0100| |
1) For spatial efficiency, instead of vertically, I noted the formula’s outcome in the truth table columns horizontally and between ||.