Quantum field theory (QFT) is the union of quantum mechanics and Einstein's special theory of relativity. QFT is the most fundamental physical theory and provides the answers to questions about the basic structure of the physical world. However, the way QFT describes the world is so different from the way our senses present it to us that it requires an overhaul of philosophic categories and arguments. How can we know the microscopic world without a satisfactory measurement theory? What are the general conditions of the world that make possible such knowledge? What are the presuppositions of physical theories? Dr. Auyang argues that the current relational - substantial debate on space - time is inadequate and the observer - observed distinction in quantum interpretations illusory because they are hampered by philosophical constructs too narrow to accommodate the complicated structures of modern philosophical theories. Modern physical theories have incorporated many presuppositions of empirical knowledge, and consequently the theories inform us about the general nature of the world and out relation to it, in addition to information about elementary particles. The conceptual structure of QFT is extracted and articulated in common- sense terms, and used to answer many longstanding philosophical questions.