[TO CHARLES N. BROWN, Locus]
Dear Charles,
April 23, 1979
This is to let you know that after four years of research and work I have
finally finished my new novel VALIS for Bantam. In the writing I collaborated
with the spirit of the 13th century Jewish Kabbalist Abraham ben Samuel
Abulafia who seizes control of me from time to time, as circumstances require.
Hope all is well with you.
Cordially,
Philip K Dick
p. 225, The Selected Letters of Philip K. Dick: 1977-1979, ed. Don Herron, Underwood Books, 1993.
The Book of Doctrines and Opinions: 
I’m planning on putting something along those lines in the acknowledgments of my dissertation.
Dear Committee,
This is to let you know that after many years of research and work I have
finally finished my dissertation entitled “A crippling Work of Heartbreaking Procrastination.” In the writing I collaborated with the spirit of the 16th century Jewish alchemist/mystic, Hayyim Vital who seizes control of me from time to time, as circumstances require, to perform elaborate internet searches on esoteric topics such as the transmutation of lead, levitation, and the best recipes for gelato.
Hope all is well with you, and my apologies for having dragged you from your long term care facilities for the defense.
Colin Wilson authored a piece on the same novel a while back;
http://www.armory.com/~mortoj/magick/newfiles/philipkdick.html
Well, I was also a bit puzzled by such revelation. But… as far Abu-al-Afiya means “Father of Health” and as far the plot of Valis is ALL about the theme of Sickness-Health (in mental dimension), one can assume – as suggested by T. Sikora – that Dick was under influence of Jerome Rothenberg & co. (vide: Abulafianism Among the
Counterculture Kabbalists by Christine A. Meilicke, Jewish Studies Quarterly 9,1 (2002) 71-101)