Consideration
Consideration is what distinguishes enforceable promises from those promises which are gratuitous. Courts will not enforce gratuitous promises but they will enforce promises which are supported by consideration. Consideration is needed to support all basic contracts, if not the agreement may not have any legal effect. Consideration means if one party is to be held to a contract, the other party must give up something in exchange. No party can have a free way out or the ability to attain something of value without providing something in exchange. In a contract there is no minimum amount of consideration required as courts presume that people will only make deals that they consider worthwhile. For example you cannot avoid liability on the ground that "I provided more than I acquired".
Capacity
The parties to a contract must have the legal capacity and competency to contract. In order to make an enforceable contact, people have to be able to understand what they are doing. The exceptions to this are:
§Minors
§The mentally incapable through drugs, alcohol or illness
§Corporations
For example corporations can make contracts as they are regarded as legal persons in their own right.
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