Ownership rights to an item cannot be obtained other than by taking for possession, by attachment, by past time, by inheritance both according to the law and according to a testament, and by transferring or hand over based on a civil event for the transfer of ownership, which is conducted by the person who has the right to act on the item.
Thus, the validity of ownership is very much determined by the validity of the way the owner obtains ownership. If the ownership comes from the transfer of ownership based on the agreement, the validity of the ownership is determined based on the validity of the agreement.