Chapter 4 explores the Western mainstream IRT, realism, neoliberal institutionalism and constructivism, as well as the English School and argues that a similar metaphysical component for all of them is individualistic rationality. It is instrumental rationality for realism and neoliberal institutionalism, and normative rationality for constructivism and the English School. Individualism constitutes the common ontological denominator while rationality provides a convenient epistemological and methodological device. Together they constitute the metaphysical component. These theories may differ greatly in terms of their substantial components, but tend to converge gradually to form a synthesis due to the similarity of their metaphysical components. As a result, we have witnessed the neo-neo synthesis between realism and neoliberal institutionalism, a rationalistic-constructivist reconciliation between rationalistic theories and Wendtian constructivism, and a transatlantic rapprochement between the American mainstream and the English School via Wendtian constructivism. Their commensurable metaphysical components make the convergence possible.