This document discusses parametricity and how types are useful in Clojure. It explains how parametricity prevents "fast and loose reasoning" and provides several examples of functions annotated with types to demonstrate theorems about their behavior. It also discusses escape hatches that can break parametricity as well as property-based testing to check functions when types alone are not sufficient. The key takeaways are that types can improve code quality, comments using types aid comprehension, and to write property tests before unit tests.