40. If it appears from the nature of the case that it was the intention of the parties to any contract that any promise contained in it should be performed by the promisor himself, such promise must be performed by the promisor. In other cases, the promisor or his representatives may employ a competent person to perform it.Illustrations (a) A promises to pay B a sum of money. A may perform this promise, either by personally paying the money to B or by causing it to be paid to B by another; and, if A dies before the time appointed for payment, his representatives must perform the promise, or employ some proper person to do so. (b) A promises to paint a picture for B. A must perform this promise personally.