 Quidditism therefore implies that laws of nature are metaphysically contingent, for, for any class of properties, there seems to be nothing standing in the way of either the possibility that they are bound up with each other by laws of nature or the possibility that they are not related at all to each other by any laws of nature. A consequence of quidditism is therefore that although, for instance, colour properties and mass properties are not actually linked with each other by any laws, they could have been so linked. Laws of nature are therefore a contingent matter. Some quidditists may go further to say that laws of nature are a type of regularity among property instances (Ramsey 1978; Lewis 1973; Earman 1984). Others may suggest that laws of nature are nomic necessitation relations between universals (Armstrong 1983; Dretske 1977; Tooley 1977).