ion of a promise being valid only when given in return
for something accepted in consideration of the promise, it follows that
the giving of the promise and of the consideration must be simultaneous.
Words of promise uttered before there is a consideration for them can be
no more than an offer; and, on the other hand, the obligation declared
in words, or inferred from acts and conduct, on the acceptance of a
consideration, is fixed at that time, and cannot be varied by subsequent
declaration, though such declarations may be material as admissions. It
was a long while, however, before this consequence was clearly
perceived. In the 18th century it was attempted, and for a time with
considerable success, to extend the range of enforceable promises
without regard to what the principles of the law would bear, in order to
satisfy a sense of natural justice. This movement was checked only
within living memory, and traces of it remain in certain apparently
anomalous rules which are indeed of little practical importance, but
which private writers, at any rate, cannot safely treat as obsolete.
However, the question of "past consideration" is too minute and
technical to be pursued here. The general result is that a binding
contract is regularly constituted by the acceptance of an offer, and at
the moment when it is accepted; and, however complicated the transaction
may be, there must always, in the theory of English law, be such a
moment in every case where a contract is formed. It also follows that
an offer before acceptance creates no duty of any kind ("A revocable
promise is unknown to our law"--Anson); which is by no means necessarily
the case in systems where the English rule of consideration is unknown.
The question what amounts to final acceptance of an offer is, on the
other hand, a question ultimately depending on common sense, and must be
treated on similar lines in all civilized countries where the business
of life is carried on in a generally similar way. The rules that an
offer is understood to be made only for a reasonable time, according to
the nature of the case, and lapses if not accepted in due time; that an
expressed revocation of an offer can take effect only if communicated to
the other party before he has accepted; that acceptance of an offer must
be according to its terms, and a conditional or qualified acceptance is
only a new proposal, and the like, may be regarded as standing on
general convenience as much