And the whole secret, power, and knowledge of their own discovery is locked within them -- they know it, feel it, have the whole thing in them.
-- THOMAS WOLFE, The Web and the Rock, 1939
The types of polygraphic interrogation we have been concerned with so far have all been designed for lie detection. We have seen that the validity of these various methods is generally unproven and that the limited evidence available indicates that this validity is modest at best. I have explained why I believe that no amount of future research and development is likely to improve much on the present, rather bleak, situation. The prospects may be better for another and fundamentally different method of polygraphic interrogation that is intended to detect, not lying, but the presence of guilty knowledge. In Chapter 20, I describe my own initial experiences with the Guilty Knowledge Test, review the evidence so far available as to its accuracy, and explain why this method seems to me to have promise as a tool of criminal investigation. Because this method is not well understood even by many polygraphers, I illustrate its application in Chapter 21, together with the theory of the Guilty Knowledge Test, and the possibilities for its future development.