This book proposes replacing the philosophical tradition inaugurated by Descartes and Locke which is inherently idealist and, says the author, prone to impoverished notions of rationality and creativity with human ontology. This latter understands human being as nothing but the complex inter-relations between its biological constitution and the natural, social and linguistic worlds in which it is embedded and the emergent properties such as conciousness, subjectivity and selfhood which arise from these. Drawing on the phenomenologies of Merleau Ponty and Heidegger but placing them in this naturalistic context the study shows how knowledge, rationality and creativity arise from within the human relationship to the world. Only on such a basis, it is argued, can the existence of a world independent of human conciousness be sustained along with the rationality of science and coherent theories of the self and truth.