This book is dedicated to my parents.
Special thanks go to the late Professor Peter Lipton, whose input, inspiration, and encouragement were invaluable in the production of this work. Professor Lipton died on 25th November 2007, one year after the publication of this book. No other person has had such a profound influence on my intellectual development. He encouraged my ideas, refined my thought, and broadened my vision. It is to him I owe my greatest philosophical debt.