When I present my research work on programming languages, people often ask me “why do you need a new programming language to solve this problem? Why not just implement it as a library?” Or, I get asked “why didn’t you implement it as an extension to {some existing language}?” In this essay I will try to make explicit some of the goals and motivations behind language design. They are formulated wholly from my own background in this area, so I might be missing some important ones I haven’t thought about.