# Are there any interesting examples of a formal system that can prove its own consistency?

## What is a consistent formal system?

A formal system is complete if for every statement of the language of the system, either the statement or its negation can be derived (i.e., proved) in the system. A formal system is consistent if there is no statement such that the statement itself and its negation are both derivable in the system.

## Can a consistent theory prove its own inconsistency?

To answer the question in the title: Yes, there are consistent theories that prove their own inconsistency.

## How do you prove a consistent ZFC?

Consistency proofs for ZFC are essentially proofs by reflection, meaning that we note, in some way or another, that since the axioms of ZFC are true, they are consistent. For example, for every finite subset A1,A2,.. An of axioms of ZFC, it is provable in ZFC that these axioms have a model, hence are consistent.

## Is Pennsylvania consistent?

Peano Arithmetic, PA, is a formal first-order theory containing constant 0, functions / (successor), +, ×, and the usual recursive identities for these functions.

## Which of the following is an example of a formal system?

Examples of formal systems include: Lambda calculus. Predicate calculus. Propositional calculus.

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## Can a formal system be inconsistent?

A formal system (deductive system, deductive theory, . . .) S is said to be inconsistent if there is a formula A of S such that A and its negation, lA, are both theorems of this system. In the opposite case, S is called consistent. A deductive system S is said to be trivial if all its formulas are theorems.

## What did Gödel prove?

Kurt Gödel’s incompleteness theorem demonstrates that mathematics contains true statements that cannot be proved. His proof achieves this by constructing paradoxical mathematical statements.

## Can an inconsistent theory have a model?

There is no such thing as an ‘inconsistent model’. One can only speak of ‘inconsistent theories’. Models are mathematically consistent because they are actual objects of the set-theoretic universe, which we assume to be contradiction-free.

## Can you be consistently inconsistent?

unchanging in nature, standard, or effect over time. However, if something were to constantly change (lets say, pi for example – no sequence of numbers in it ever repeats), you could say that it is consistently inconsistent, because it has an unchanging nature to be an inconsistent string of numbers.

## Is PA con Pa consistent?

Consistent, ω-inconsistent theories

Now, assuming PA is really consistent, it follows that PA + ¬Con(PA) is also consistent, for if it were not, then PA would prove Con(PA) (reductio), contradicting Gödel’s second incompleteness theorem. However, PA + ¬Con(PA) is not ω-consistent.

## Is peano arithmetic consistent?

The simplest proof that Peano arithmetic is consistent goes like this: Peano arithmetic has a model (namely the standard natural numbers) and is therefore consistent. This proof is easy to formalize in ZFC, so it’s certainly a proof by the ordinary standards of everyday mathematics.