Pleading rule

However desirable it may be to admit in evidence, on the general issue in an action of covenant on a policy of insurance, every thing which may avoid the contract, or lessen the damages, as is done in actions on the case, this court does not know that it possesses the power of changing the law of pleading, or to admit of evidence inconsistent with the forms which it has prescribed. No rule on this subject is more inflexible than that, in actions on deeds, all special matter of defence must be pleaded.