The wonderful thing about dynamic typing is it lets you express anything that is computable. And type systems don't — type systems are typically decidable, and they restrict you to a subset. People who favor static type systems say it's fine, it's good enough; all the interesting programs you want to write will work as types. But that's ridiculous — once you have a type system, you don't even know what interesting programs are there.

Software Engineering Radio Episode 140: Newspeak and Pluggable Types with Gilad Bracha