We define an inference system to capture explanations based on causal statements, using an ontology in the form of an IS-A hierarchy. We first introduce a simple logical language which makes it possible to express that a fact causes another fact and that a fact explains another fact. We present a set of formal inference patterns from causal statements to explanation statements. These patterns exhibit ontological premises that are argued to be essential in deducing explanation statements. We provide an inference system that captures the patterns discussed.