# Propositional Logic System

(Redirected from Propositional Logic)

## References

### 2015

• (Wikipedia, 2015) ⇒ http://en.wikipedia.org/wiki/Propositional_calculus Retrieved:2015-2-25.
• … The following is an example of a very simple inference within the scope of propositional logic:

:Premise 1: If it's raining then it's cloudy.

:Premise 2: It's raining.

:Conclusion: It's cloudy.

Both premises and the conclusions are propositions. The premises are taken for granted and then with the application of modus ponens (an inference rule) the conclusion follows.

As propositional logic is not concerned with the structure of propositions beyond the point where they can't be decomposed anymore by logical connectives, this inference can be restated replacing those atomic statements with statement letters, which are interpreted as variables representing statements:

:Premise 1: $P \to Q$ :Premise 2: $P$ :Conclusion: $Q$ The same can be stated succinctly in the following way: : $P \to Q, P \vdash Q$ When is interpreted as “It's raining” and as “it's cloudy” the above symbolic expressions can be seen to exactly correspond with the original expression in natural language. Not only that, but they will also correspond with any other inference of this form, which will be valid on the same basis that this inference is.

Propositional logic may be studied through a formal system in which formulas of a formal language may be interpreted to represent propositions. A system of inference rules and axioms allows certain formulas to be derived. These derived formulas are called theorems and may be interpreted to be true propositions. A constructed sequence of such formulas is known as a derivation or proof and the last formula of the sequence is the theorem. The derivation may be interpreted as proof of the proposition represented by the theorem.

When a formal system is used to represent formal logic, only statement letters are represented directly. The natural language propositions that arise when they're interpreted are outside the scope of the system, and the relation between the formal system and its interpretation is likewise outside the formal system itself.

Usually in truth-functional propositional logic, formulas are interpreted as having either a truth value of true or a truth value of false.Truth-functional propositional logic and systems isomorphic to it, are considered to be zeroth-order logic.