What is the test for an individual to be bound as to the terms of a contract?

In the same case, Mr. Justice Blackburn said, at p. 607, But I have more difficulty about the second point raised in the case. I apprehend that if one of the parties intends to make a contract on one set of terms, and the other intends to make a contract on another set of terms, or, as it is sometimes expressed, if the parties are not ad idem, there is no contract, unless the circumstances are such as to preclude one of the parties from denying that he has agreed to the terms of the other. The rule of law is that stated in Freeman v. Cooke [citation deleted]. If, whatever a man's real intention may be, he so conducts himself that a reasonable man would believe that he was assenting to the terms proposed by the other party, and that other party upon that belief enters into the contract with him, the man thus conducting himself would be equally bound as if he had intended to agreed to the other party's terms.