The current article was begun during my visit to Oxford University in 1978–79. I would like to recognize the hospitality of that institution and to express my indebtedness to David Pears who gave very generously of his time and advice during the early stages of this research. I would also like to express my gratitude to Jaakko Hintikka who endured long discussions on the present work and who is responsible for numerous improvements in the last section.