"Start with God and man's immortal soul and you've lost every reader except those who believe in God and man's immortal soul.
Start with B. F. Skinner and man decreed as organism who learns everything he does by operant conditioning and you've lost every reader who knows there is more to it than that and that Skinner has explained nothing. Skinner explains everything about man except for what makes him human, for example, language and his refusal to behave like an organism in an environment."
Personally, I adhere to the first group who begin with "God and man's immortal soul", but I appreciate Percy's desire to find a common ground from which to work and see if we might not come to an appreciation, if not agreement, on the unique status and nature of that species to which we belong. At least, perhaps, we can even agree on what being a human being is not: decreed organisms bound by conditions who respond according to behavioral patterns, etc. For a very interesting literary vignette that runs parallel to this see the beginning of the second chapter of Charles Dickens's Hard Times, where Thomas Gradgrind, the teacher, probes "Girl number twenty" to provide a definition of a horse. I'm grateful to Jon McCord for this wonderful reference.