In this chapter, we describe a formal semantics for SL and for QL. The word ‘semantics’ comes from the greek word for ‘mark’ and means ‘related to meaning.’ So a formal semantics will be a mathematical account of meaning in the formal language.
A formal, logical language is built from two kinds of elements: logical symbols and non-logical symbols. Connectives (like ‘&’) and quantiﬁers (like ‘∀’) are logical symbols, because their meaning is speciﬁed within the formal language. When writing a symbolization key, you are not allowed to change the meaning of the logical symbols. You cannot say, for instance, that the ‘¬’ symbol will mean ‘not’ in one argument and ‘perhaps’ in another. The ‘¬’ symbol always means logical negation. It is used to translate the English language word ‘not’, but it is a symbol of a formal language and is deﬁned by its truth conditions.
The sentence letters in SL are non-logical symbols, because their meaning is not deﬁned by the logical structure of SL. When we translate an argument from English to SL, for example, the sentence letter M does not have its meaning ﬁxed in advance; instead, we provide a symbolization key that says how M should be interpreted in that argument. In QL, the predicates and constants are non-logical symbols.
In translating from English to a formal language, we provided symbolization keys which were interpretations of all the non-logical symbols we used in the translation. An interpretation gives a meaning to all the non-logical elements of the language.
It is possible to provide diﬀerent interpretations that make no formal diﬀerence. In SL, for example, we might say that D means ‘Today is Tuesday’; we might say instead that D means ‘Today is the day after Monday.’ These are two diﬀerent interpretations, because they use diﬀerent English sentences for the meaning of D. Yet, formally, there is no diﬀerence between them. All that matters once we have symbolized these sentences is whether they are true or false. In order to characterize what makes a diﬀerence in the formal language, we need to know what makes sentences true or false. For this, we need a formal characterization of truth.
When we gave deﬁnitions for a sentence of SL and for a sentence of QL, we distinguished between the object language and the metalanguage. The object language is the language that we are talking about: either SL or QL. The metalanguage is the language that we use to talk about the object language: English, supplemented with some mathematical jargon. It will be important to keep this distinction in mind.