Agreements may lack none of the essentials of a contract, and, if within the Statute of Frauds, may be in the form that the law requires, but, nevertheless, be subject to affirmative defences which render them unenforceable. Such defences may be divided into two classes - those which operate as soon as they arise to prevent any legal obligation from arising, and those which enable one or both parties to avoid their obligations by appropriate manifestation of intention. The division cannot be made wholly exact. Illegal agreements for instance cannot be placed as a whole in any one category, but for purpose of dividing the chapters of a treatise, the classification is sufficiently accurate, and such defences as do not ordinarily of their own force immediately deprive a contract of legal validity or even prevent an agreement from having legal force, will first be considered. Perhaps the most important of these is fraud.