The generalization which is implied by that argument is as follows: When and if a proposition P is logically necessary, the sole guarantee of its necessity-the licence for saying “ You must treat P as necessary” — is some form of legislation by the company concerned. What we call this piece of legislation depends upon our context and our purpose: in some contexts it will be called a convention, in others an agreement, in others a ruling, in others an understanding, and so on. The only essential is explicit or implicit agreement that it has the force of law. The act or acts of legislation involved may however be more or less complicated and more or less widely accepted outside the limits of the immediate company. Sometimes, for example, the ruling will be a simple one, applicable only temporarily to a particular statement-token (as in the case of “ ‘This is red all over’ entails ‘This is not green’ ” ) or to a definition adopted in a particular argument (e.g. “ We shall say that such and such a formula constitutes the scope of an occurrence of such and such a symbol” ). Sometimes there will be explicit or implicit acceptance of the rules which are explicitly formalized in a parti cular book. And usually there will be an implicit agreement to adopt as necessary the rules which the present company agrees to be accepted by the whole company of (sane) philosophers throughout the world.