Metamath is a computer-assiste proof checker. It has no specific logic embedded and can simply be regarded as a device to apply inference rules to formulas. Simplicity is the master concept in the design of Metamath: the language of Metamath, employed to state the definitions, axioms, inference rules, and theorems, is only composed of a handful of keywords and all the proofs are checked using one simple algorithm based on the substitution of variables.