The meaning of the term "evolvability" has long been ambiguous within evolutionary biology. Because of this, interpreting and understanding statements made in the literature of the field such as "x is more evolvable than y" has been very difficult. In this paper I propose a novel analysis of evolvability using conditional probability that provides a "common currency" for discussions in the field. This account allows us to easily represent common statements about evolvability currently seen in the literature. In addition, it provides a framework via which we can assess the presence and nature of causal relationships between small scale features of the world and higher level evolutionary patterns. This has important implications for future research into evolvability and macroevolutionary patterns in the tree of life more generally.