# Notion of Safeness of Inference Rules

Safeness is a name for property of logical inference rules.
It is in use now
for more than a dozen of years at least. It is not yet an established standard
such as soundness and completeness of a set of inference rules.
Roughly speaking, a rule is safe if its inverse rule is sound.
More precisely, a rule is sound iff the validity of its preconditions
implies the validity of its conclusion;
a rule is safe iff the invalidity of one of its preconditions
implies the invalidity of its conclusion.
For reductive inference systems, soundness means that a goal
is solved if we can solve the sub-goals it is reduced to.
Safeness means that if one of the sub-goals is unsolvable,
the goal is unsolvable, too.
Safeness is useful for disproving theorems.
If we disprove a sub-goal generated from a goal or from one of its
transitive sub-goals, then, if all reduction steps are safe,
we know that the original goal was invalid already.
Lecture Notes on Logic, incl.
Safeness

This is file p/safeness.html on Claus-Peter Wirth's web site.

Last modified 2005/05/02/11.15