權利瑕疵 自始權利瑕疵 嗣後權利瑕疵 權利無缺 權利存在 他人之物之買賣 自始客觀不能 評價矛盾 舉重以明輕 類推適用 Obligations of non-defect of right Obligation of existence of right Obligation of non-enforceable right Sale of another's thing Primarily impossible prestation Analog application Contradiction in valuation