He was an outspoken skeptic of the idea of molecular assemblers, as advocated by K. Eric Drexler. His main scientific objections, which he termed the "fat fingers problem" and the "sticky fingers problem", argued against the feasibility of molecular assemblers being able to precisely select and place individual atoms. He also believed that Drexlers speculations about apocalyptic dangers of molecular assemblers threatened the public support for development of nanotechnology.  He debated Drexler in an exchange of letters which were published in Chemical & Engineering News' as a point-counterpoint feature.