A set of replicas is diverse to the extent that all implement the same functionality but differ in their implementation details. Diverse replicas are less prone to having vulnerabilities in common, because attacks typically depend on memory layout and/or instruction-sequence specifics. Recent work advocates using mechanical means, such as program rewriting, to create such diversity. A correspondence between the specific transformations being employed and the attacks they defend against is often provided, but little has been said about the overall effectiveness of diversity per se in defending against attacks. With this broader goal in mind, we here give a precise characterization of attacks, applicable to viewing diversity as a defense, and also show how mechanically-generated diversity compares to a well-understood defense, strong typing.