Function definition of Clojure built-ins: def vs defn

Question

Why are some (many) of Clojure's built-in functions defined using def, not defn?

I know that defn is a shortcut for def, but I'm wondering if there's some other reason such as efficiency/readability/style considerations/bootstrapping.

Answer 1

Read further down in clojure/core.clj - defn doesn't exist yet at the top, because that file is building the language as it goes. After several hundred lines, defn is written, and functions after that are defined with defn.