| Transformation rules |
|---|
| Propositional calculus |
|
Rules of inference Rules of replacement Commutativity Distributivity Double negation De Morgan's laws Transposition Material implication Exportation Tautology |
| Predicate logic |
| Universal generalization Universal instantiation Existential generalization Existential instantiation |
A rule of inference is a rule justifying a logical step from hypothesis to conclusion. There are several rules of inference which utilize the existential quantifier.
Existential introduction (∃I) concludes that, if the propositional function is known to be true for a particular element of the domain of discourse, then it must be true that there exists an element for which the proposition function is true. Symbolically,
The reasoning behind existential elimination (∃E) is as follows: If it is given that there exists an element for which the proposition function is true, and if a conclusion can be reached by giving that element an arbitrary name, that conclusion is necessarily true, as long as it does not contain the name. Symbolically, for an arbitrary c and for a proposition Q in which c does not appear:
must be true for all values of c over the same domain X; else, the logic does not follow: If c is not arbitrary, and is instead a specific element of the domain of discourse, then stating P(c) might unjustifiably give more information about that object.
Read more about this topic: Existential Quantification, Properties
Famous quotes containing the words rules of, rules and/or inference:
“When I hear the hypercritical quarreling about grammar and style, the position of the particles, etc., etc., stretching or contracting every speaker to certain rules of theirs ... I see that they forget that the first requisite and rule is that expression shall be vital and natural, as much as the voice of a brute or an interjection: first of all, mother tongue; and last of all, artificial or father tongue. Essentially your truest poetic sentence is as free and lawless as a lamb’s bleat.”
—Henry David Thoreau (1817–1862)
“The new grammar of race is constructed in a way that George Orwell would have appreciated, because its rules make some ideas impossible to express—unless, of course, one wants to be called a racist.”
—Stephen Carter (b. 1954)
“Rules and particular inferences alike are justified by being brought into agreement with each other. A rule is amended if it yields an inference we are unwilling to accept; an inference is rejected if it violates a rule we are unwilling to amend. The process of justification is the delicate one of making mutual adjustments between rules and accepted inferences; and in the agreement achieved lies the only justification needed for either.”
—Nelson Goodman (b. 1906)