“What is design?” seems like an increasingly silly question to me. When people were asking “What is science?” and “What is not?” the impetus seems to have been quite different. They wanted to know how to construct reliable knowledge, who we should believe and who not, in effect. And consequently, who should receive special status and privilege and who shouldn’t. We don’t seem to have these problems in design.
First of all, “design” is such a broad field, spanning people pursuing such differing goals, that it almost moves to the same category of questions as “What is love?”. I mean, it’s an interesting question to ponder and perhaps to try and discuss philosophically, but it’s not going to be of practical consequence anytime soon.
What problem would answering the question “What is design?” solve? One answer might be that having a clear and correct answer would help us in the activity of designing. But whether science ‘works’ is a logical, fundamental question; whether a design method ‘works’ is a question of efficiency. When it comes to effectiveness, we do not have a problem. People are perfectly capable of designing things.