Nicod's axiom

Nicod's axiom (named after Jean Nicod) is an axiom in propositional calculus that can be used as a sole wff in a two-axiom formalization of zeroth-order logic.

The axiom states the following always has a true truth value.

To utilize this axiom, Nicod made a rule of inference, called Nicod's modus ponens.

1. φ

2. (φ ⊼ (χ ⊼ ψ))

∴ ψ

In 1931, Mordechaj Wajsberg found an adequate, and easier-to-work-with alternative.