If mental assent is a necessary element of contract, and if offer and acceptance are important merely as proving this mental assent, as the ultimate fact to be established, a failure of either party to express his actual intent is of vital importance as indicating that no assent existed though the appearance of assent might be established by an offer and acceptance. That is, if A said, " I offer to sell my horse " for a certain price when he meant he would sell his cow, and the offeree accepted the offer intending to buy the horse, there would be no contract because no mutual assent.
21 Equity Jurisprudence (13th ed.), 108.
311 Columbia Law Rev. 199 (Foulke).
4 An illustration of the danger of understanding mistake in the sense of act rather than in the act of state of mind is illustrated by the language in Meek v. Hurst, 223 Mo. 688, 696, 122 S. W. 1022, and Wolz v. Venard, 253
Mo. 67, 83, 161 S. W. 760, intimating that a mistake of a scrivener in writing a deed was of no consequence unless he was the agent of both parties, for otherwise it is said the mistake (apparently meaning the erroneous writing) would not be that of both parties. It is obvious that the mental error of both does not depend on the agency of the scrivener.
On the other hand, if the expression of assent by the parties is what constitutes a contract, there are the essential dements in the case in question. As has previously been shown,6 it seems clear that whatever difference there may be in the expressions of courts,6 the actual results of the decisions favor the objective theory. This is sufficiently indicated in the decisions at law on the parol evidence rule (which is necessarily based on the assumption that the written expression of the parties is controlling) and by the decisions on unwritten contracts enforcing liability although there may have been misunderstanding between the parties,7 for under the subjective view any misunderstanding between the parties would be fatal unless an estoppel could be made out. Doubtless the law is generally expressed in terms of subjective assent, rather than of objective expressions, the latter being said to be "evidence" of the former, as for example in the so-called parol evidence rule; but when it is established that this is no rule of evidence8 the whole subjective theory falls to the ground. Under the guise of conclusive presumptions of mental assent from external acts, the law has been so built up that it can be now expressed accurately only by saying that the elements requisite for the formation of a contract are exclusively external.