Carl Sagan asks “What’s the difference between physical and “psychological” addiction?

In a recently discovered letter dating back to 1990 from Carl Sagan to Dr. Arnold S. Trebach, the then president of the Drug Policy Foundation,  American astronomer, astrophysicist, cosmologist, author, science popularizer Sagan asks “What’s the difference between physical and “psychological” addiction, and how is my attachment to music or my wife different from “psychological” addiction?