Consider "Sam is sad" and "Sam exemplifies the property of being sad". The second sentence mentions a property and predicates the relation of exemplification. It belongs to a large class of sentences which mention such formal objects as propositions, states of affairs, facts, concepts and sets and predicate formal properties such as the truth of propositions, the obtaining of states of affairs and relations such as falling under concepts and being members of sets. The first sentence belongs to a distinct class of sentences in which only non-formal objects are mentioned and only non-formal properties and relations are predicated. We can, it seems, infer validly from the first sentence to the second. They are also equivalent. And Sam exemplifies the property of sadness because Sam is sad. What is the relation between inference, equivalence and explanation in the case of our two sentences and in analogous cases? What right have we to assume that there are formal objects?