What is the principle of implied or expressed in the context of a deposit under a contract?

The principle quoted in many of the Canadian authorities is that of Fry, L.J. in Howe v. Smith, [1884] 27 Ch. D. 89, at p. 101: Money paid as a deposit must, I conceive, be paid on some terms implied or expressed. In this case no terms are expressed, and we must therefore inquire what terms are to be implied. The terms most naturally to be implied appear to me in the case of money paid on the signing of a contract to be that in the event of the contract being performed it shall be brought into account, but if the contract is not performed by the payer it shall remain the property of the payee. It is not merely a part payment, but is then also an earnest to bind the bargain so entered into, and creates by the fear of its forfeiture a motive in the payer to perform the rest of the contract.