It seems formerly to have been supposed that an offer must be accepted instantaneously, or that it ceased to exist.1 This idea was doubtless based on the theory that the offer was only important as evidence of a state of mind. Since then in the nature of things an offer could be evidence of the offerer's mental attitude only at or about the time he spoke, it was essential that the offer and acceptance should be contemporaneous.2

The necessity of contemporaneous existence of offer and acceptance has sometimes been met by the fiction of a re-

1 The often cited case of Cooke v. Oxley, 3 T. R. 663, shows this conception. There the offer was by its terms to remain open until 4 o'clock, and an acceptance was made before 4 o'clock. The court held that there was no contract either at the time when the offer was made, nor at the later time, because at neither time did both parties assent to the bargain. The same assumption that the acceptance must be contemporaneous with the offer even though the offer expressly state that it shall remain open for a given time, is to be found in Head v. Diggon, 3 Man. Sec. Ry. 97. 2 See infra, Sec.Sec. 95, 1536, 1637. ' lation back of the acceptance to the offer.3 It is of course true that offer and acceptance must exist at the same time, but the offer is not merely evidence of a state of mind; it is an element in the formation of a contract irrespective of the offerer's mental attitude, and may continue effective in spite of a change in that attitude, It is, therefore, unnecessary to resort to a fiction that the acceptance relates back, in order to support the validity of a contract made without practically simultaneous expressions of assent.4 It is the only accurate way to express the matter to say that the offer continues till the acceptance, rather than that the offer relates back to the acceptance. It is now well settled that an offer will continue in force subject to the limitations hereafter stated.5

The ways in which an offer may be terminated other than by an acceptance, which changes it into a contractual obligation, are: (1) Rejection by the offeree; (2) Expiration of a time stated in the offer itself as the limit allowed for acceptance; (3) Expiration of a reasonable time, if no time limit is fixed by the offer; (4) Revocation by the offeror; (5) Death or insanity of either party.