When we defined the interpretation of propositions, we insisted that a proposition has a truth value that is either *true* or *false*. We also noted that propositions build only a part of all the strings which are syntactically correct among other strings which can be formulated in a given formal language $L$. But how about all the other strings, for which we cannot assign a truth value? Important types of such statements are *paradoxes*.

Let a formal language $L$ be given, in which the valuation function $[[]]_I$ of $PL0$ (law of excluded middle) holds. A **paradox** is a string $s\in L$, for which the interpretation $[[s]]_I=undefined,$ i.e. for which it is not possible to assign a truth value, and which apparently contradicts itself.

| | | | | created: 2018-01-07 00:09:46 | modified: 2020-05-04 18:57:20 | by: *bookofproofs* | references: [7838]

[7838] **Kohar, Richard**: “Basic Discrete Mathematics, Logic, Set Theory & Probability”, World Scientific, 2016