Two FOPL w s are said to be equivalent if for all interpretations, both of them evaluate to the same truth
value. Table 3.17 presents some important logical identities involving FOPL w s. Here P and Q are arbi-
trary w s and P[x] represents a w P that involves the variable x.
e identities in Row 6 should be considered carefully. First of all, we must appreciate that all vari-
ables are essentially ‘dummy’ variables in the sense that any symbol can be used for them, so long as they
do not ‘collide’ with one another. For example, (∀x) P [x] is equivalent to (∀y) P [y]. Similarly, (∀x) P
[x] • (∀y) Q [y] is equivalent to (∀x) P [x] • (∀x) ...
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