Supreme Court Justice John Paul Stevens Retiring

When talking about Supreme Court nominees it is often repeated that the person has to be a lawyer. Every possible nominee that is being mentioned is a lawyer.

IANAL, but there is nothing that I can find in the constitution to support this. Article 2. Section 2. [the president] "shall nominate, and by and with the Advice and Consent of the Senate, shall appoint... Judges of the supreme Court". This is the only qualification I can find in the constitution for a member of the Supreme Court.

It would be a good idea to have someone with other knowledge and experience on the court. How about a governor or a legislator? What would be good qualifications for a justice who is not a lawyer?