That is a difficult question to answer because I’ve worked in so many fields. I’ve worked in five different fields. Now which are the most important? In some ways, the development of the biological species concept, which I did not invent, but I certainly was the person that brought it to general knowledge. And the whole field of new systematics — I really am the one who developed it — was one of my major contributions. There’s another one, which is still not yet realized by almost anybody. I have done more, I believe, for the development of a philosophy of biology than any other person.
Academy of Achievement interview, 5 April 2001 source ↗