R2d § 19
Conduct as Manifestation of Assent
(1) The manifestation of assent may be made wholly or partly by written or spoken words or by other acts or by failure to act. (2) The conduct of a party is not effective as a manifestation of his assent unless he intends to engage in the conduct and knows or has reason to know that the other party may infer from his conduct that he assents. (3) The conduct of a party may manifest assent even though he does not in fact assent. In such cases a resulting contract may be voidable because of fraud, duress, mistake, or other invalidating cause.
Professor's notes
Elements: (1) assent may be manifested by written or spoken words OR by other acts or failures to act; (2) conduct is not effective as a manifestation unless the actor intends to engage in the conduct AND knows or has reason to know the other party may infer assent; (3) conduct can be effective even when the actor does not in fact assent.
Lucy v. Zehmer shows subsection (3) in action: Zehmer's conduct (signing, sliding the paper) bound him even though he claims he did not subjectively assent.
Common misunderstanding: students think silence cannot manifest assent. Often true, but § 19 plus § 69 leave room: prior dealings and accepted benefit can convert silence into acceptance.
Cases that operationalize this rule
Text
R2d § 19. Conduct as Manifestation of Assent.
(1) The manifestation of assent may be made wholly or partly by written or spoken words or by other acts or by failure to act.
(2) The conduct of a party is not effective as a manifestation of his assent unless he intends to engage in the conduct and knows or has reason to know that the other party may infer from his conduct that he assents.
(3) The conduct of a party may manifest assent even though he does not in fact assent. In such cases a resulting contract may be voidable because of fraud, duress, mistake, or other invalidating cause.