什么是对价呢？
English law does not enforce gratuitous promises, i.e. promises given for no return. For a simple promise to be enforceable it is necessary that the person to whom the promise was made, the promisee, should have done something in return for the promise.
That something done in return for a promise is consideration and can therefore be understood as the price paid for a promise.
如果简单的说，“对价”就是在合同中买卖方口中所说的价钱。比如，我想从您那里买商品过来，我们之间所说的价钱就是对价。
Types of consideration对价的分类：
Executory consideration(尚未履行的兑价；双边合同中-Bilateral contract)
Is the promise to perform an action at some future time.
Once party to a contractual agreement may pay money to another on the understanding that the latter will perform some act for them in the future.
Or alternatively they might provide an immediate benefit for the other party on the understanding that the latter will provide a reciprocal benefit in the future.
Contract may also be made solely on the basis of an exchange of promises as to future action, without the need for any present action.
Executed consideration(已履行的对价；单边合同中-Unilateral contract)
Refers to consideration that has actually been carried out, thus making the promise enforceable.
In the case of unilateral contract, where the offerer promises something in return for the offeree’s doing something, the original promise only becomes enforceable when the offeror has actually performance the required act.
The action requested does not have to be performance but once it is done then the original promise becomes legally enforceable.
Past consideration(过去的对价；已经消失意义的对价)
Does not actually count as valid consideration; therefore no agreement resting on past consideration is legally enforceable.
Normally consideration is provided either at the time of the creation of a contract or at a later date. In the case of past consideration, however, the action done is performed bf the promised. Such prior action is not deemed sufficient to support the later promise.
