This book addresses the challenge of scepticism facing knowledge and justification, by attempting a reconciliation of two divergent rationalist epistemological positions: "Descartes' foundationalism” and "Popper's fallibilism.” Like Kant’s "Antinomies of Reason," these positions demonstrate the equally rational but conflictual results of applying to the universe of pure thought the categories or criteria of reason proper to the universe of sensible perception, or experience. The harmony envisaged is accomplished through the use of Kant’s “Synthetic a Priori Judgments." This succeeds in rendering Descartes' foundationalism and Popper's fallibilism "co-ordinate perspectives," within the rationalist tradition in Epistemology. The word “co-ordinate” implies the unification of opposites, or the organization of contraries into a unity. The resultant theory from the reconciliation attempted in this book is “fallibilist foundationalism,” which is proposed as a plausible, rationalist epistemological theory of defeasible justification, with innovative potentials for Science Education in Nigeria. Philosophers, and others who engage in epistemological debates, will find this book very useful.