This book invites to interpret Karl Popper under the light of his ethical attitudes. In the first part a previously unpublished text by Popper is reproduced and commented which is most relevant to acquire a new insight on Popper's philosophy and should be taken into account in any future interpretation of it. In the second part, under the light of the new insight, the ethical roots of Popper's theory of knowledge are analysed, jointly with the meaning and reach of his fallibilism.