If the contract of the agent is in fact within his authority, the principal is liable thereon, without reference to any facts creating estoppel, or to the knowledge possessed by the adversary party of the facts that make the principal liable.1 Payment to an authorized agent discharges the debt paid.2 The principal is liable for the acts of his agent within the scope of his authority even if the existence of the principal is not disclosed.3 Thus payment to the agent of an undisclosed principal discharges the debt.4 So if the contract does not purport to bind the real principal, but the agent,5 or a third party who does not consent thereto,6 the real principal is liable thereon.