Propositional logic, also known as sentential logic, is that branch of logic that studies ways of combining or altering statements or propositions to form more complicated statements or propositions. Joining two simpler propositions with the word "and" is one common way of combining statements. When two statements are joined together with "and", the complex statement formed by them is true if and only if both the component statements are true. Because of this, an argument of the following form is logically valid:
One of the fundamental concerns in philosophy is that with truth. But what kind of things are true?—what is truth a property of? The traditional answer is that it is propositions which are true or false, where propositions are to be distinguished from sentences. There are four main arguments for denying that it is sentences which are true or false and for introducing the apparatus of propositions to stand as the bearers of truth.
Firstly, "sentence" is a grammatical concept and not all grammatically well-formed sentences appear to express anything which is capable of being true or false: for example, "All green ideas sleep furiously". This sentence is grammatically well-formed, but clearly meaningless. Some sentences, we shall say, do not express any proposition at all.
Secondly, some sentences are ambiguous. We normally explain this by saying that one sentence (string of words) is capable of expressing more than one proposition: for example, "Flying aeroplanes can be dangerous", which can mean either that being a pilot can be a dangerous activity, or that aeroplanes can be dangerous when they are flying about in the sky.
Thirdly, different sentences can have the same meaning. We would normally think of translation from one language to another to be possible because sentences from different languages can express the same proposition: for example, "It is raining", "Il pleut" and "Es regnet".
Fourthly, we tend to think that there is some meaning in common between the indicative, interrogative and imperative sentences in the table below, and this is normally explained by differentiating their assertoric force from their propositional content.
Propositionsare distinct from sentences. For one thing, proposition, which is an expression of judgment, is made up of concepts while sentence is made up ofwords. In a declarative sentence, the proposition is not the sentence itself, but that which is expressed or asserted, which is either true or false. In other words, a sentence is not the bearer of truth or falsehood. These are properties of propositions.
Definition: A proposition or statement is a sentence which is either true or false.
Definition:If a proposition is true, then we say its truth value is true, and if a proposition is false, we say its truth value is false. Are these propositions?