Phillip K. Dick, author of SF novels that have been made into numerous films ("Blade Runner", "Minority Report", "Paycheck") denied the existence of a certain person. Who was that person?

Stanislaw Lem is a Polish SF writer, author of "Solaris". In 1974 Philip K. Dick (probably suffering from schizophrenia) sent the letter to the FBI stating that "Lem is probably a composite committee rather than an individual, since he writes in several styles and sometimes reads foreign, to him, languages (...) these people are Marxists (...) and a threat to our whole field of science fiction"