In the first part of this paper, I argue against the view that laws of nature are contingent, by attacking a necessary condition for its truth within the framework of a conception of laws as relations between universals. I try to show that there is no independent reason to think that universals have an essence independent of their nomological properties. However, such a non-qualitative essence is required to make sense of the idea that different laws link the same universals in (...) different possible worlds. In the second part, I give a positive argument for the necessity of at least some laws of nature, by showing with the example of a paradigmatic law of association that it consists in an internal relation between two universals which are determinables of the same class of determinates, where this relation is essential to both. Furthermore, I show that the necessity of laws of association could be accommodated within David Lewis' Humean metaphysics, but that it is incompatible with David Armstrong's combinatorialism. (shrink)