Sentence:  Although no provision of the Civil Code lays down the principle of invocability, it is nonetheless unanimously admitted and is found in substantive law in two forms: the <WSD>opposability</WSD> of the contract to third parties, and the <WSD>opposability</WSD> of the contract by third parties. 
Meaning of the word marked with <WSD> tags:  The condition or quality of being opposable (capable of being resisted).