He may admit the plaintiff's allegations but plead other facts "to avoid their effect." This is called the plea of confession and avoidance.

These pleas are on the merits of the case, and are called _pleas in bar_.

There are other pleas of this kind.
"Pleas in bar, except the general issue, may give rise to counter pleas" introduced by the parties alternately.
But the issue may be one of law instead of fact, and the defendant may enter a _demurrer_, claiming that the matters alleged are not sufficient in law to sustain the action.
Evidence .-- Some of the fundamental principles or rules which govern the taking of evidence and the weighing of testimony may properly appear here.
These rules are designed to exclude all irrelevant matter and to secure the best proof that can be had.
1.

_Witnesses must be competent_.

That is, in general, they must be able to understand the nature and solemnity of an oath.