# Are there impossible non-classical logics?

## What is non classical logic in philosophy?

Non-classical logics (and sometimes alternative logics) are formal systems that differ in a significant way from standard logical systems such as propositional and predicate logic. There are several ways in which this is done, including by way of extensions, deviations, and variations.

## What is classical philosophical logic?

Typically, a logic consists of a formal or informal language together with a deductive system and/or a model-theoretic semantics. The language has components that correspond to a part of a natural language like English or Greek.

## How many kinds of logic are there?

four

The four main types of logic are: Informal logic: Uses deductive and inductive reasoning to make arguments. Formal logic: Uses syllogisms to make inferences. Symbolic logic: Uses symbols to accurately map out valid and invalid arguments.

## Is modal logic classical?

Every regular modal logic is classical, and every normal modal logic is regular and hence classical.

## What is a Kripke frame?

A Kripke frame or modal frame is a pair. , where W is a (possibly empty) set, and R is a binary relation on W. Elements of W are called nodes or worlds, and R is known as the accessibility relation.

## Is modal logic useful?

An understanding of modal logic is particularly valuable in the formal analysis of philosophical argument, where expressions from the modal family are both common and confusing. Modal logic also has important applications in computer science.

## Is second order logic complete?

Several deductive systems can be used for second-order logic, although none can be complete for the standard semantics (see below). Each of these systems is sound, which means any sentence they can be used to prove is logically valid in the appropriate semantics.

## Is fol complete?

Perhaps most significantly, first-order logic is complete, and can be fully formalized (in the sense that a sentence is derivable from the axioms just in case it holds in all models). First-order logic moreover satisfies both compactness and the downward Löwenheim-Skolem property; so it has a tractable model theory.

## Why is second-order logic incomplete?

Theorem: 2nd order logic is incomplete: 1) The set T of theorems of 2nd order logic is effectively enumerable. 2) The set V of valid sentences of 2nd order logic is not effectively enumerable. 3) Thus, by Lemma One, V is not a subset of T.

## Is predicate logic second-order?

In mathematical logic, a second-order predicate is a predicate that takes a first-order predicate as an argument. Compare higher-order predicate. The idea of second order predication was introduced by the German mathematician and philosopher Frege.

## Is first-order logic consistent?

By PROPOSITION 3.5 we know that a set of first-order formulae T is consistent if and only if it has a model, i.e., there is a model M such that M N T. So, in order to prove for example that the axioms of Set Theory are consistent we only have to find a single model in which all these axioms hold.

## Is first-order logic Axiomatizable?

Their axiomatization of first order logic will typically contain an axiom of the form ∀xϕ1→ϕ1[y/x] with varying qualifications on what the term y is allowed to be, along the lines of ‘y is free for x in ϕ1’.

## Is first-order logic extensional?

Extensions of First Order Logic is a book on mathematical logic. It was written by María Manzano, and published in 1996 by the Cambridge University Press as volume 19 of their book series Cambridge Tracts in Theoretical Computer Science.

## What is the difference between extensional and intensional?

intension and extension, in logic, correlative words that indicate the reference of a term or concept: “intension” indicates the internal content of a term or concept that constitutes its formal definition; and “extension” indicates its range of applicability by naming the particular objects that it denotes.

## Is predicate logic complete?

Truth-functional propositional logic and first-order predicate logic are semantically complete, but not syntactically complete (for example, the propositional logic statement consisting of a single propositional variable A is not a theorem, and neither is its negation).