# Hilbert system

*is an infrequently used term for a physical system described by a C*-algebra.*

In a Hilbert-style deduction system, a **formal deduction** is a finite sequence of formulas in which each formula is either an axiom or is obtained from previous formulas by a rule of inference. These formal deductions are meant to mirror natural-language proofs, although they are far more detailed.

The first four logical axiom schemes allow (together with modus ponens) for the manipulation of logical connectives.

Intuitionistic logic is achieved by adding axioms P4i and P5i to positive implicational logic, or by adding axiom P5i to minimal logic. Both P4i and P5i are theorems of classical propositional logic.

With a second rule of uniform substitution (US), we can change each of these axiom schemes into a single axiom, replacing each schematic variable by some propositional variable that isn't mentioned in any axiom to get what we call the substitutional axiomatisation. Both formalisations have variables, but where the one-rule axiomatisation has schematic variables that are outside the logic's language, the substitutional axiomatisation uses propositional variables that do the same work by expressing the idea of a variable ranging over formulae with a rule that uses substitution.

The next three logical axiom schemes provide ways to add, manipulate, and remove universal quantifiers.

The final axiom schemes are required to work with formulas involving the equality symbol.

Because Hilbert-style systems have very few deduction rules, it is common to prove **metatheorems** that show that additional deduction rules add no deductive power, in the sense that a deduction using the new deduction rules can be converted into a deduction using only the original deduction rules.

Following are several theorems in propositional logic, along with their proofs (or links to these proofs in other articles). Note that since (P1) itself can be proved using the other axioms, in fact (P2), (P3) and (P4) suffice for proving all these theorems.