In an actio rei vindicatio, the owner need only allege and prove ownership and that the defendant is holding the property - the onus is on the defendant to establish any right to continue to hold the property against the owner. A mere custom or practice, even if established, does not constitute a defence to an actio rei vindicatio unless it creates a contractual or other legally enforceable right. The existence of a past practice of allowing retiring directors to purchase their vehicles does not create an enforceable obligation on the company to offer the vehicle for sale, nor does it create a right of retention in the absence of an actual offer and acceptance constituting a binding contract. A legitimate expectation based on past practice, without more, does not justify retention of property against the owner's wishes.