ABSTRACT
Some of the notorious difficulties in connexion with modal concepts concern the relation of what people shall call higher order modalities to lower order modalities. It is a 'received truth' of modal logic that if a proposition is true, then it is also possibly true. If a proposition is possibly true, then the proposition in question is also possibly possibly true, and if a proposition is not possibly true, then the proposition in question is also possibly not possibly true. The two identities are consistent with what people shall call the 'received system' of modal logic. This chapter considers physical models which are 'isomorphic' with the above geometrical models of propositional logic and various systems of modal logic. It shows that the 'logical behaviour' of natural processes which are, in a defined sense, additive with regard to their effects is governed by the same formal rules as the concept of possibility.
