In earlier posts, we explained why consistency is an important property of axiomatic systems and discussed relative proofs of consistency, in which the proof of consistency of a system is based on the assumption that another axiomatic system is consistent. In this post, we introduce absolute proofs of consistency that do not make any assumptions about any other axiomatic system. Apparently,

David Hilbert was the first to study and propose such proofs, according to Nagel and Newman's book (Section III, page 26).

Recall that an axiomatic system is consistent if it cannot derive both a theorem and its negation. What do we mean by the negation of a theorem? Let's take, as a simple example, the theorem: "6 is divisible by 3." Its negation is simply the following statement: "6 is not divisible by 3" or equivalently "It is not the case that 6 is divisible by 3." This second formulation of the negation, although less elegant in English, is preferable because the negation is added to the front of the original theorem. In a formal system, negation is handled by simply adding a symbol for the phrase "it is not the case that." Several symbols have been used for negation, such as ~ and ¬ . We'll use the latter here. So, if

* T* is any theorem in some formal system, then the formula ¬

*T *is the negation of

*T*.

Remember that our formal system

*FS* did not have a symbol for negation. So we will extend

*FS* into a new formal system called

*FSN* (for

*FS* with

* N*egation), whose alphabet is {

*E*,

* 0*,

*1*, ¬ }.

*FSN* has exactly one axiom, namely the same as A1 in

* FS*.

*FSN* also has the same inference rules as

*FS*, namely IR1 and IR2. But it has one additional inference rule that uses negation. Here is the full description of

*FSN*: